I've descended Alex Barr (via Larch Mt. & Haines) on 23s. It's doable, and there's a spooky abandoned(?) house along the way, but I wouldn't recommend it to anyone. For comparison, I consider Saltzman (the one in the West Hills) just fine, if muddy.
I have looked into Palmer Mill, but was not able to find it from the highway or Larch Mountain. It might be a myth.
Update: thanks to this Strava Multiple Ride Mapper
, it looks like I turned around just shy of Palmer Mill on Brower Road, right where it makes a final turn east (and into gravel).