With your permission, I will try to give you some succinct

answers to the questions you’ve posed, although I must say that much better answers are sprinkled all over this forum. There are two concepts one must grasp and keep in mind: (1) the concept of
time; and, (2) the concept of
sequence both of which we will get back to later on.
In John’s gospel, the author states that, “In the beginning was the Word…” As we read other parts of the New Testament, we come to discover that Christ is [also] the Word. Further, He is, He that “…am who am.” These indicate that He was always there, with the Father (God) and what is more, He, together with the Father and the Holy Spirit, not only exists, but also, IS Existence
per se.
Now, we know that for natural creatures (such as we humans), it is in the
nature of said creatures to exist as singular, unified ones, although, said creatures are body (corporeal matter) and soul (incorporeal matter) in some sort of
unity. How we are unified is quite mysterious even though there are lots of books written on the subject. But, at least it does give us a peek, if you will, at the mystery of the Trinity.
It is the
nature of God to exist as more than one person (which we can explore later on), as it is the
nature of natural creatures to exist as ensouled matter. I know that some will try to disqualify the concept of the “nature of a thing”, but, it is in the nature of our rational minds to strive to understand things, and being, by extracting their universals.
There cannot be an infinite regress because our natural and scientific experience with the world and the universe tells us that it is finite, for one thing. It is certainly huge, but, a huge finite number (called a “
transfinite”) is still a finite. If it is finite, then there must be a
beginning, or First Mover.
If, on the other hand, the universe is infinite, then there would be no First mover and thus no secondary, or, tertiary, or, quaternary, etc., movers. “Motion”, in this regard, really means “
change”. It can be as simple as a change of positions (local motion), or, more complex, as “coming to be.” But, in general, it means the more complex “change”.
Think of the alternative - infinite reality - like this: there would be no bounds, no beginning, no ending. It would be as a circle. Now, in such a circle, with an infinite number of things (if such was at all possible) there would be nothing to change. Everything would already be in
act. There would be no
potency to act. But, that would be absurd. It would defy our senses. Remember, we experience
change. That means, we experience things at rest (before change), or, in potency, and after change, or, in actuality. At some point, on an infinite chain, all things would be in act (fully actualized), thus, change (motion) would no longer occur and it would come to a complete stop.
I must stop now, but, either I will resume later, or, someone else will take up the gauntlet and do a better job than me. If not, I’ll continue later this evening. The need to do this is out of my control!
jd