Yes, both are used. I think you're about right with the colloquial/formal distinction.

I wouldn't say the phrase "if it weren't for" is actually formal, though; just more so than "if it wasn't for".

It's like "If I were you", I think, in that you can use it without any awareness of the grammar behind it.

Speaking for myself (but probably meaning much the same as CM2DD), "were" is correct in more formal contexts, but many of us also say "If it were't for Brexit" in informal conversation.

#4 No, I mean that the subjunctive is gradually falling out of use, so the subjunctive is usually more formal. But there are some set phrases like "If I were you" or "If it weren't for X" that use the subjunctive, but have stayed in use (even non-formally) because they are more like a single building block; people don't think about the grammar - it's like a fossil word. "If it weren't for X" is more formal than "if it wasn't for X", but I don't think it is only used in formal English.

Mildly OT: The subjunctive hasn't fallen from use quite as much in AE as it has from BE.