Turns out the cause of the remaining front ISO play after tightening the bolts was the upper headset bearing being worn. Replaced with a new $11 bearing and its tight as new. Lesson is, don't pound over rough terrain with a loose headset like and idiot (me).
I also swapped the swanky one (titanium) bolt 3T ARX-II stem for a beefier Zipp Carbon Speed stem with two bolts. A few rides on it now and it doesn't loosen up anymore. So using an single bolt road stem wasn't a good idea for rugged offroad use. In retrospect, that makes sense (duh).