They work pretty similar as long as you use good compressionless brake housing with the mech. The main problems with mech are if the brakes are low quality or someone uses spongey housing. Hydro feels maybe a bit stiffer and stronger if comparing two good setups, but not a huge deal.

Hydros do self-tighten as the pads wear out, since the amount the pads back out when you release the lever depends on how far they had pressed in against the rotor. *Usually* this isn't a big deal within a given event, though. Maybe make sure there's a barrel adjuster accessible from the handlebars on the mech brake housing if you're unsure...

Yeah. With well-adjusted cantis with good pads, shouldn't make much real difference in the dry. Discs do offer better modulation when mucky, since they're better at sweeping their braking surface.