Nice work, better be safe than sorry on the support since it will be very heavy.
1) I would insulate under the mass too as the temps stay pretty high and you probably want your heat moving up and not down.
2) You cannot use stovepipe for the riser, it will burn out.
3) you can use metal on the outside of the riser where the temperature will be cooler--people commonly used small sized oil drums--33gallon i think
4) stove pipe for the riser or for the horizontal run? They will hold up in the mass but not in the riser--"metal is doomed"
5) longer is more efficient, but it really depends on how much length you already have, what is the length with 2 runs vs 3 runs--including 90degree elbows at 5' each?