The reason that the AI does not build good minesweepers is because the AI Research only covers the techs that the specific race uses. The AI may, at a later time, move into other areas of research but only after they get the tech that is specified in the txt file. I have seen them build level 5 minesweepers, but they don't build enough of them to deal with a 100 mine minefield. BUT they will keep sending them one after another at the minefield and will eventually sweep all the mines (not very efficient).
You are right about "SEIV's mines feature "gimmicky" anyway, I didn't think I'd miss them (and I don't)." if the AI does not use them correctly, then it wastes time and resources on building them.
I feel the same about the intel part of the game. I guess I've just seen to many games where the AI steals your tech and then all their "Friends" have it a few turns later. Then again I can see where there would be races that exisit to steal technology! Personel choice I guess.