There was a good session on UL1838 and NEC at the AOLP conference. You've got a couple of these wrong, you can use smaller than 12 gauge wire in an LVL system, you just need to make sure the fuse or breaker is sized correctly for it. If you have a 25A breaker, you need 12ga wire.
The burial depth is actually 6" below grade. Grade is the dirt level, like you said, not mulch.
That 18" under hardscape is if it's not in a conduit. You can go 6" if you use a rigid conduit.