1) Get the stock ROM here
. Yours would be the number 7. Since you seem to still have the original firmware, the kernel should be the same. Also, if you ever unroot your phone, flash the stock ROM first
and only then unroot it. Since you won't be able to flash the stock ROM if you unroot it first.
2) I'm sorry but I don't understand your question. Try to phrase it better, please. Anyway, I assume you probably already read this, but here's what I find to be the best and simplest tutorial for rooting here: link
3) Yes, you can - but only if rooted. adb commands, on the other hand, will be usable even if the phone isn't rooted. (Thus you can root via adb too.)