I think you'll find bulk commercial RO
water is much cheaper per gallon than bottled distilled. In my area, 32 vs 99 cents per gallon (Walmart/Culligen prices). I've never seen a commercial distilled water dispenser in my area. IMO
compared to 0 is a negligible difference. I don't even change my RO
media until it hits a consistent tds
of 5 or more.
Regarding the canister media, anything will trap debris including your rock (sump or otherwise), macroalgae, overflow, etc. IMO
it won't collect enough to be a nitrate issue if maintained at a reasonable level minues the mechanical filter media.