Punishment tiers:
First time you get away with a warning pop-up for a minor offense
10 game chat restriction
25 game chat restriction
14 day ban
Permanent ban
Number of reports doesn't matter.
Just that they are valid reports.
If you get a certain ammount of valid reports they get reviewed.
When reviewed only your actions are taken into account and no one elses in the game.
If someone provoked you and you started flaming each other only you will be judged.
That is why riot recommends muting toxic people and reporting them after the game, you might say something that could potentially be punishable.
Your punishments will allways go up by one tier unless for racism, homophobia, extreme harassment etc...
Then they might skip to ban tiers.
After you get a punishment your account is in bad standing for ~3 months ( the last i checked but could be less ), and after that it drops by one tier.
While your account is in bad standing, reports against you will be taken more seriously and reviewed faster so you have to be carefull.
After you drop all the tiers your account is in good standing.
Positive feedbacks to my knowledge do not necessarily outweigh negative.
Once you are banned you will be presented with chat logs the punishment was based on.
You can dispute a ban by submiting a ticket to support, but those are only reversed if it was riots mistake.
There is no returning a rightfully banned account.
Also check this link for some official statements
https://support.riotgames.com/hc/en-us
I think i covered all of your questions.
Hope this helps you.