If the plug did tighten, then the threads can either move freely or are already stripped, I'd just back it out and cross your fingers. I've done more spark plugs than I can count in aluminum heads, at first I thought the threads would pull out on half of them, but I havn't had a single one pull any out. The funny thing is, the only time I did have the threads pull out was on a cast iron head
I know what you're saying about aluminum, but it definitely has it's upsides too. A Cavalier with the cast iron 2.4l handles and rides worse than the same car with the 2.2 Ecotec. Not much, but it is noticeable if you really get going
. You also have more HP to accelerate because you're not moving all that weight with you. The latest version of the 2.4 Quad4 makes 5 HP more than the 2.2 Ecotec, yet is slower.
Use anti-seize, change fluids on time, and use a torque wrench... pretty much all you need to do with aluminum.