Just an update. Ended up booking Premium Economy, leaving on a Thursday, return on a Tuesday. Then called up Exec Plat desk, and surprisingly, got confirmed upgrade on the return trip from Madrid->DFW (still economy DFW->AUS, but exit row, etc., so fine, and I often get upgraded on that leg anyway just due to status.) I was really surprised, because NOTHING I could find (AA website OR ExpertFlyer.com) showed any availability, and I check every day. Waitlisted on AUS->LHR, hopefully it comes through, the rep seemed to indicate it wasn't hopeless.
So, although at this point I'm still in Premium Economy going east, I did get an upgrade to business coming back west, which is the longest flight of the whole trip (11 hrs).
Bonus: when I called and she confirmed the availability, I got her to change my original reservation from Premium Economy to regular-old economy, then apply the upgrade (still gets you to business.) Saved a quick $300 just for asking, which I thought was cool.
On the last question, I had enough miles to book 2 RT in business, but only with MileSaaver fares, which limit your options. All of them involved BA flights and terrible schedules, and additionally they would have cost me something like $1300 in fees to book my "free" flights. That, the crappy times, and having to use all those miles, I just couldn't see my way to doing that.