Perhaps Spinoza made up a straw man, with the intention of propelling the reader to a revelatory conclusion. That would indeed make complex to identify the "sense" of the word god: is it the literal Spinozan god which is immanent or the mystical god which is to be deduced from the literature?

This is just pantheism. In a monistic system like Spinozas, reality would trivially satisfy the criteria for an impersonal God referent in that it is self subsistent, and simultaneously is the source and creator of things.

Spinoza's formulation is "God or Nature", that is, God = Nature.

Some are of the opinion that he used the term God as an expedient or subterfuge. One could not at that time speak as freely as we do today. Others think that he was truly pious, a believer, only not a believer in the traditional characterizations of God.

You are right that there is no referent or bearer of the name God. Perhaps that should be your focus.

I am writing a text about Spinozas metaphysical philosophy — marcolobo8

This should be obvious but just in case you are writing it as assignment for university, the most credible reference would be your Prof. or Assistant. So asking them is always recommended.

The paradox however, is the fact that Spinoza in no way shape or form, correlates anything religious/supernatural with this One Substance/God. He even rejects free will as he sees Causality to be included in every process whatsoever. — marcolobo8

Where do you get this from? I mean you make it sound like a trivial obvious point. However I personally would contest it.

So why use the word "God" — marcolobo8

First of I support the notion.

Some are of the opinion that he used the term God as an expedient or subterfuge. One could not at that time speak as freely as we do today. Others think that he was truly pious, a believer, only not a believer in the traditional characterizations of God. — Fooloso4

Second the general understanding of God as Substance is not really that new. In platonic Terms it would be the Idea of Ideas, which is similar to how Spinoza constructs his "God" starting of with Substances and arriving at there only being one Substance.
Historically the Newplatonians and early Christians are said to have "married" the Idea of Ideas with Personhood/Agency creating the "usual" concept of god.
The main difference I see is not the existing of such a Substance/Idea and rather the fact that there is only one Substance making everything somehow part of god differentiating from scholastic positions that emphasize the separateness of God from other Substances by using a model with multiple Substances. Connected to this one can argue that the need for agency diminishes however to come back to my contesting your point, it doesn't necessarily remove consciousness as such from the equation. If you look at Def. 3 It defines Substances as being understood (by whom) through itself.
Since there is only one Substance the by whom question could be answered as
a) by itself (substance)
b) by an Attribute (Def. 4 Attributes are what the Mind (whose mind) understands for Substance to be the constituting fact of it's essence(up to go translation important is the mind part))
c) by a Modus. But Modus also contains a part "through witch it is understood" that contains consciousness
As you see all 3 Basic termini use to a certain degree a comprehending. Where you want to locate this is one question but for me it isn't that for of a stretch to understand at least some parts of consciousness to be part of Substance which 1) creates closer proximity of Spinozas God and the classical god and 2) Is a religious/spiritual view.

Is God an actual object of reference which the sign "God" refers to? — marcolobo8

Is it a concept and not an object? — marcolobo8

I am not to used to Frege and not sure what you exactly are referring to when stating "object" or "concept". The answer to this question is highly dependent on the usage of those terms. As I understand them I don't see why the terms should be exclusive.

It seems to me that there really is no clear definition/shared sense of what "God" means. — marcolobo8

I don't know how you derive at this conclusion or what you are trying to express. Since after all there clearly is a Definition of God (Def.6) and since it's a tautological system theres no reason to care about the shared sense that people ascribe to the term "God". So you seemingly seem to speak about the conventional "God" but why you are considering this when you are trying to analyze Spinozas understanding of the term "God" via Frege is something I don't understand.
Are you trying to answer the questions in the Frege part for both usages of the term "God"?

The first thing to consider, in my mind, is the focus in On Sense and Reference upon distinguishing the form of identity of a=a from a=b. Toward that end, Frege framed the term 'sign' with some constraints:

"It is clear from the context that by 'sign' and 'name' I have here understood any designation representing a proper name, which thus has as its reference a definite object (this word taken in the widest range), but not a concept or relation, which shall be discussed further in another article. (On Concept and Object) The designation of a single object can also consist of several words or other signs. For brevity, let every such designation be called a proper name."

This constraint makes me wonder if Frege would not consider Spinoza's use of 'God' as more of a "Concept and Object" problem. However that may be, It is important to consider Spinoza's definition of God in the context of his own limits of expression. Ethics, Definition 6:

"By God, I mean a being absolutely infinite - that is, a substance consisting in infinite attributes, of which each expresses eternal and infinite essentiality.
Explanation - I say absolutely infinite, not infinite after its kind; for, of a thing infinite only after its kind, infinite attributes may be denied; but that which is absolutely infinite, contains in its essence whatever expresses reality, and involves no negation."

In so far as Spinoza speaks of us as "being in God", the above argues against your statement that "Spinoza in no way shape or form, correlates anything religious/supernatural with this One Substance/God." All the ways available to us to inquire, seek the causes of, or plan for ourselves cannot speak in the register of this God.

The paradox however, is the fact that Spinoza in no way shape or form, correlates anything religious/supernatural with this One Substance/God. He even rejects free will as he sees Causality to be included in every process whatsoever.
So why use the word "God", when in fact he is describing what a scientist etc. would see as scientific and rational states/processes of nature. — marcolobo8

Spinoza, who was a rationalist, proposed this belief called substance monism. When you thoroughly think of it, we never really had free will — because a god determined our free will, from there it could get so logically and rationally complex. Entirely, Spinoza did not only define his theory of substance monism to be made out of God, but that the things are filled by God. Everything that defines something or someone essentially or accidentally, God designed that through his substance and willingness to do so. That includes: purpose, characteristics, and so forth. It wasn't only made out of him, it was also because of him.

For example, take a rock. A rock has different characteristics however its purpose is not definitive, as it varies by living interference. Where the rock's purposelessness resides, is where God placed that to be, living interference occurs and then therefore, gives it purpose. Our free will was determined by God, but that's not to say our free will is not much of, free will. So Spinoza did not really reject causality, but merely specified it to be Godly decided — Godly determined. (I would gladly accept if I am wrong. If there is somewhere, Spinoza directly or even indirectly rejects this, I am keen to admit.)

So a scientist = scientist, and a scientist is not God, yet only made out of him.

It seems to me that there really is no clear definition/shared sense of what "God" means. — marcolobo8

I think it is because it wasn't given, what we know of God is only approximate. God, suppose he exists, is infinity. Infinity is a concept, but God is not — since he is infinite, it would be a safe assumption to make, that he is ever-growing. After all, he is: omnitemporal and omnipresent. So like the universe, it is ever-growing, it abides to infinity, but that doesn't mean it isn't somewhat an object.

Is God an actual object of reference which the sign "God" refers to? — marcolobo8

To be consistent, I did say that our knowledge of particularly, God, by biblical and realistic principle, is only approximate. So necessary conditions, which would be a first step to deciding God as a reference or sense, has nothing to do with God. The nomenclature of properties for God is void; there are no intrinsic essential or accidental attributes that observes him. God is indefinable.

Spinoza does clearly disregard God has any particular traits which characterise a creator or particular forces the world. It's the major distinction of his philosophy. Substance (God) is that which is infinite and cannot be subject to change. That which is beyond creation and destruction, a universal constant of all. God, that which is the same, regardless of what the world does or how it acts.

A theistic God could never hope to achieve this status. They are mere beings who change, alter and may even die/be destroyed. They are not infinite. They change not creating at one point, to creating at another. They are not constant.

With respect to the classical God, Spinoza is the ultimate heretic. His point is that an infinite God, the constant, beyond change and destruction, cannot possibly exist at all. To exist means to be finite, ephemeral, possible (as opposed to necessary), changeable and destructible, everything an infinite God is not.

The infinity of God can only be given in God's non-existence (in the sense a being who take actions, thinks,creates, etc.).