Put it simple.
It is not possible to rectify which.
Now think of the "which" as an ordinary noun like "apple" in your mind. Hypothetically. Yes. The "which" is the object of the verb, "rectify".
And the "it" represents the same thing as in "it is nice to meet you".
Regarding your further questions,
Underneath PB: 1 That works in a way, sometimes. But sometimes not.
Usually they say, "it is nice to meet you". "Meeting you is nice" rather than "to meet you is nice". Although "to meet... nice" is grammatically correct. And you can rarely hear people say "meeting you is nice", because it sounds like you are a judge now judging if this event, "meeting you", is whether nice or not, it's a stiff and stupid statement rather than a common compliment when you first meet someone.
2 It's the relative pronoun (here referring to "mistakes"), whatever subjects of the (main) sentence.
3 sounds no problem at all. I mean good in this context only, can also say "Mistakes... and... rectify them", or "... and they are not possible to rectify".
Questions under Mr. Sakaverde's answer:
1 Yes you can.
2 Yes you uh... morphologically can. I'd consider replacing the last verb because "rectify" is for "mistakes", but when "which" represents the initial statement, better be something like, "..., which is not possible to avoid".