Thanks for everyone's comments. It's a nice bonus of playing the Champion Match to get these. Here are my 2 cents.
Game 1, Scrying Pool, Black Market, Wall: I obviously agree that it was a big mistake to not buy Ambassador. There, I forgot that I still had so many coppers because I used Upgrade on other stuff. (I didn't want to get into the attacking too much at that point because I felt ahead - and there was Forge left in the BM deck that I revealed on my first play.)
Commentators seem to agree that you can pretty much ignore Wall there, which is definitely true for the early game. But at some point you really have to ask yourself whether building more is worth the negative Wall points, especially when there's no Alt-VP. So I think that taking Cache for effectively two coins was better than getting some alternative sources of coins. Similarly, I wanted to just get those Provinces over getting some Grand Markets and build more.
Side note: not playing all treasures on BM with Watchtower in hand was the convenient thing to do when I had already drawn my deck already as playing treasures individually with BM is a pain.
Game 2, Urchin, Governor, Salt the Earth: Salting the Province on my last turn was the big mistake there that cost me the win. I was too greedy / bad at deck tracking to miss out on the 4th Governor mid-game. I guess, I was too convinced that going Urchin/Urchin was the better thing to do that I was overconfident and didn't play it safely enough.
Game 3, Peasant, Apothecary, Hag/YW: I stand by Potion over Sea Hag. Rabid's teacher choices were bad of course, which made it easier for me. But I think that getting 2 Peasants on their way and getting Young Witch later is better.
Game 4, Soothsayer, Rebuild, Miser, Artificer: I just thought that opening Miser was a really bad choice for Rabid especially after me opening Soothsayer. I wasn't too committed to go for Rebuild from the beginning but at some point I thought that there's no way for me to lose it, if I go for Rebuild and end it quickly.
Game 5, Ironworks, Highway, Alms (or at least that's how we saw the board apparently
): this game was very weird. Part of the "problem" I think is that we just reenforced each other in our oppinion that we're doing the right thing, because the opponent did the same. Although, I think you clearly want to get to Ironworking and Alms'ing 5 cost cards. I think that IW+Silver opening is fine. Alternative is Silver/Silver, but I don't see the game plan from there: you have a good shot at 2 five costs on turn 3+4 but then you don't have an IW and the Silver is rather hurting you in the long run. Rabid gave up the chance for hitting 5 by not IWing Silver - I agree that's wrong and I don't think that I did that. I think I was not very lucky to collide Highway with IW afterwards.
I'm not sure what the conclusion is: people didn't like our rush, but it didn't seem so wrong while playing it.
Game 6 Caravan, Soothsayer, Save: I didn't believe in City Quarter there, even though Save helps it. I think an early City Quarter collided well for Rabid that also won him the Curse split. I should have gotten a Horse Traders at some point, although I didn't know where. My Festival was a bit unuseful in the later game as I didn't draw it with my money and would have had to forgo a Province in order to Save it. In the end, I was very lucky to win that game as Rabid didn't save Vault.