Will I get permabanned on a second offence if my first offence was a 14 day?

Yes. If you do not go enough time+games played without bad behavior to drop down a punishment level, your next punishment will likely be a permanent suspension.

Depends on how long it has been since your 14 day suspension, otherwise the answer is yes.

If you go about 3-6 months (varies by how much you play, and if you get validly reported) without a punishment, your punishment tier will drop, so that your next punishment would be a 14 day ban. If you go an additional 3-6 months after that, then your punishment level could be reduced further back down to a 25 game chat restriction. 3-6 months after that, and your account will be sparkling clean.

After a 14 day ban, however, the bar for toxicity is set quite low, so it's best to be super careful of what you say for a while. I advise to only typing about the future gameplay. We should ward up there, push that wave then get dragon, Victor is strong: next fight lets not group together and maybe buy some MR, ect.

Pointing out mistakes, linking kill scores, asking for reports, saying that you've muted someone, ping spamming, or saying that you'll report someone are all things that you should avoid completely.