This is an interesting topic. I'm 6' tall and have shorter than normal legs for my height with a 32 inseam, and tend to ride in a squat knees out / elbows out position. I found riding 29er trail bikes on steeper terrain resulted in some terrifying butt buzz situations. I've since had a ton of time on the Intense M29 and the Aurum HSP 29er with no butt buzz issues. However I haven't ridden a ton of super steep trails on the Aurum or Intense, most of my riding was in the Whistler Bike Park on those bikes. I think your point about the slacker head angle and longer front center does a lot to mitigate the butt buzz I was experiencing on the trail bikes. The rearward axle path on the Aurum probably also helps since the tire moves backwards almost as much as it moves vertically under compression.
As for the offset cups, it's a great reach adjustment on the Aurum HSP. I ran the bike in the longest setting and loved the geometry on the L/XL for my height. I'd recommend going with the bigger bike and running it in the shorter setting if you find it too long.
This probably doesn't help you at all, and its counter intuitive. But I just ordered and built up a 27.5 Norco Range trail bike for 2019 as my personal bike, having the choice of both tire sizes. And I'd buy the Aurum HSP 29er (although I haven't ridden the 27.5 variant ... but I REALLY enjoyed the HSP 29er).