I'm glad to hear your desire to reform! It's certainly very refreshing in this forum, and I wish you all the best :)
On the topic of accidental repeat offenses, there is a period of time (I don't know how long, probably a few weeks or months) where your account is essentially placed on parole, and if you step too far out of line in that period, you run a very real risk of being permabanned. A big part of Riot being able to ban people resides in the chat logs. If you clean up your speech, it'll be much harder for anyone to feasibly report you with any results.
We all have bad games, we all sometimes flame our teammates, and we all sometimes get frustrated when someone on our team isn't doing well. That's alright, we're only human, and Riot understands that. It's only when it goes excessively out of line when you need to worry. My suggestion is make very frequent use of the mute button. If you fail a gank and die, mute your laner you tried to gank for. If you get caught out, or your team gets caught out and lose the fight as a result of you not being there, mute your whole team. The most triggering part about the game generally tends to be the members of your team, rather than your own play. If you focus on your own play, and not the play or speech of those on your team, you'll find you not only win more games and improve, but become less toxic as a result.
If you really are committed to reforming, you will probably have no problem at all. A good skill to learn is to recognize when you're losing badly, and to try to mitigate any further losses. This helps in not only preventing any games from looking like feeding, but also to potentially salvage some games you got behind in. Some common signs Rioters look for when scanning an account for intentional feeding are purchasing multiple of the same item, selling your current items to do so, and cs totals. If your cs increases by very little (like 1 or 2) over the course of 10 minutes, but you managed to die 8 times during that period, then it's very possible you will be investigated further.
I hate to sound all cheesy and what not, but if you try your best and stay committed, I'm sure you'll turn out ok!
