Recently, I've gotten into low-priority queue for leaving around 2-3 matches in a row due to internet instability.
Too bad. Fix your internet. Those guys aren't going to let you go even when your internet is bad.
What is the threshold where you will get low priority?
Quote from NA LoL:
The first time a player leaves a game, they’ll see a pop-up explaining that leaving is not okay and then have to manually agree to not leave future games. If they continue to ruin other players’ experiences, they’ll enter lower-priority matchmaking for a number of games.
Basically it means that when you see a pop-up after a game saying that you have to agree that you won't leave anymore games, that's the threshold. That is the last warning. From my experience, not more that 5 games.
I've heard that if you leave too many matches, you can get temporarily or permanently banned.
At what stage of time do we get banned?
Depends on your frequency of getting banned. The chances are much higher when you get banned, for example, 6 times in a very short time (6 bans in 12 games), than being banned in a long time (6 times in 120 games).
If I get permanently banned, is it possible to get unbanned?
if you get reported by other players too much, can you get banned, or muted in the chat or whatsoever?
Yes. You will be reported every time you break a rule and you can (not will) be punished. The effect of the ban can vary, depending on the severity.
What is the penalty for getting reported too much?
You will get chat restricted if people reports you for being toxic numerous times.