“Secondly, why should anyone assume that God was around before the Big Bang”
I think that would fall under the argument of cause and effect, or the idea of motion. As some philosophers argue, everything in the universe is in motion (circulation of the planets, stars, atoms, protons, etc). We understand through simple observation that in order for something to move in our physical world (exp: a ball), it must be moved. A ball sitting on an even floor does not start moving unless it is pushed (by a force, a person, the wind, etc).
With this movement (the effect) needing a push (a cause), a philosopher or Christian may come to the conclusion that the universe must have a Universal Mover/Pusher, or as some call, the First Cause.
That would be one role, and seeing that the Big Bang, if following these day-to-day observations of cause and effect, it would also need a first cause. We don’t witness anything material creating itself from non-existence into existence, so we have to assume something is causing the effect of creation. One would argue The Big Bang has a first cause; or a pusher, that isn’t material because it would be illogical for a material’s first cause to also be material, especially if we’re talking about something that’s supposed to be the beginning of all materials/matter. That material pusher would only need another pusher before that!
One argument to why a God (a Divine Being, with intellect, and not of this material world) would work as a First Cause, or the Pusher, is that this God would be all-existing and not of this world. The reason why a God wouldn’t need a first cause, is because He doesn’t exists in this material universe. God is not a product of time, matter, space, and light because He would have created time, matter, space, and light (and logically, a creator cannot be dependent on its creation in order to exist).
We Observe: Matter decays. If all things are made of matter, than all things must also one day decay (Your Chair, Your Body, The Earth, The Sun, etc).
We Observe: Things do not move on their own. If you see a ball rolling your way, you automatically assume someone or something pushed it. It’s illogical to assume the ball merely was bored and decided to roll itself.
We Observe: God is NOT of this world. God cannot be made up and subjected to matter because, if He created it, it would be illogical to assume He’s subject to it. A chef who makes a cake is obviously not made up of his cake, and so the inventor is not made up of the invention. It’s illogical to explain the chef in terms of his cake, as it’s illogical to explain the inventor in terms of his/her invention.
We Conclude: Knowing God (or The First Cause, or The Universal Pusher) is not of this world, and not made up of matter and material, we know He doesn’t need to decay like matter. Living on a plane/heaven made up of spirit, it’s understanding that He has always existed, and if this is so (not needing to be pushed and not needing to be decaying material), you would not need to assume there’s a “Big Bang” to God, or a First Cause to Him, or His own Pusher. He simply is All-Existing, All-Eternal, because he’s not subjected to his inventions (time, matter, etc). Add to that Him being Intelligent, and Him possessing Free Will, and you understand how He moves himself (as we move ourselves) and why He would create the universe to begin with (His intelligence).
For this reason, all material things (as mentioned above), would stem from a first cause, and this first cause could understandably be a Divine Being existing on a Heavenly/Divine/Non-Materialistic heaven/plane (non-material as in not being made up of matter, which be know matter as not eternal as it decomposes and dies).
Of course this is a very brief explanation, but hopefully the point comes across.
Hope that helps explain number 2 for you.