Matter is energy. The property of mass results from the interaction of one field with the Higgs field. The energy always existed.
"1.) Where did all the matter in the universe come from?"
The energy always existed. The appearence of this universe is can be seen as a kind of phase transformation. Like a bubble of water vapor forming in a pot of boiling water.
"6.) If this matter has always existed, then why isn't it equally possible that God has always existed?"
One can examine the universe, because it presents itself to be seen. Nothing is hidden. If there is a god, he also would present himself for examination.
"Since it would seem equally possible that a.) God and matter could both have always existed and since it would also seem equally possible that"
If something can be observed, it obviously exists. It's the same with a person. If something can't be observed, there is no reason to assume it does.
"b.) God and matter could have brought themeslves into existence"
A person can't bring themselves into existence. That's illogical, where the cause causes itself.
"9.) Based upon the aforementioned that both a and b are equally possible (a 50/50 situation), atheists should rather be agnostics and say that a Supreme Being (God) may exist and that they just don't know if he does.
They are not equally possible, or probable. If something can't be seen, it's extremely improbable that it exists. If what can be observed is self sufficient, i's even more improbable that something claimed, such as the arbitrary force of a spirit, or god is in effect.
There's only one person that ever showed up in history and claimed to be god. An examination of his claims and what he said is all one has regarding the decision of whether his claims are true, or not. The physics are self sufficient, meaning no unseen, outside force is required to provide a complete rational explaination for all that's observed.
The person that claimed to be god was only concerned with the relationship between himself and all the others that were in his image and likeness, and the relationships they had amongst themselves. The claim that he was the first cause for all of what he claimed to have done will forever be buried and hidden in the dual of purely physical cause. He said no physical sign would be found. Matt 12:39
He answered, "A wicked and adulterous generation asks for a miraculous sign! But none will be given it except the sign of the prophet Jonah." I believe he caused the phase transition, which resulted in this universe. Only, because of what He said otherwise. This transition itself stands as a possibility of a happening that occured entirely on it's own.
The sign of the prophet Jonah is the Holy Spirit.