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1. I still do not regret opening double Amb. Should I?
Yes. I'd open Masq/Potion. Buying a wandering minstell and a scrying pool on turns 3/4 is quite strong.
Depending on what opponent does you can add more trashing later. The price you're paying (not buying strong cards for a while) is not worth the investment here.
2. On what turn should I have gotten Potion myself (provided the answer to 1. is "no")?
This problematic question is exactly why I'd take masq right away.
3. It is clear that I started to screw around with a lot of cards I should not have favored over SP. Ignoring these mistakes, should I have tried to Amb Provinces sooner once I recognized my lead (ignoring the knowlegde I could be pinned later)?
Hmm... it's not so much any of these, but it's more that you need to make a plan around turn 16. This plan could be
a) buy a curse, flood your opponent.
b) buy provinces, end the game.
c) set up the pin.
I think your lead is sufficient that if you chose any of these plans, they'd lead you to victory. But what you're doing - the combination of a) and b) - that is very inconsistent. If you want to flood, flood full force. The goal is to cripple your opponent, you don't need Provinces for that. If you want to pile out the provinces, do it with everything you've got. I can see donating one or two of them to your opponent actually work out here.
4. Should you aim for the pin here or is that situational depending on how you come out of the Amb war with your engine?
It's situational / not that relevant what you aim for, but it doesn't really depend on how you come out of the amb war either. You either come out on top - and then you can choose whatever you like - or you lose it, and then you're toast.