1. Is it possible to create a DSB
, adding sand over dolomite?
Not sure what dolomite is, but if it is sugar fine then yes. If it is coarse like gravel then he needs to remove it.
2. Is it possible to have a DSB
with an underground filter in use?
3. If using live sand ordered online, is there a chance of an ammonia spike as one might see from LR
Possibly. It's not necessary though. He can use dry sand, building up to 4" over time (1/2"-1" at a time). It will be "live" in a few weeks and be populated with worms a few weeks later (he has LR