I don't think Rebuild games are more tactical than regular games, I just think they are less strategical and that may make tactics relatively more important for winning, but that also happens with shuffle luck being extremely important in Rebuild games. I think Rebuild mirrors are as interesting as Gardens/IW rush mirrors, so it would be good if they appear with the same really low percentage, not 5%.
I would like to back the idea to return the cards to the supply instead of trashing. However, it is important to notice that it will definitely affect itself by replenishing Duchies, so even mirror games are going to be affected, is not just a matter of leaving VP for the non-Rebuild player.
On the "how to play Rebuild" issue, I am surprised that the best company for Rebuild was not mentioned: Scheme. I would probably still open Silver/Silver equivalent or Silver/good Sifter because $5 is super important and Scheme does not help, but then I would get Scheme's at any <5. Maybe one Estate if I am out and there are plenty of Duchies.