Q1:
Yes, spanning tree is the right solution
for layer 2 redundancy. I'd recommend linking
the two 2810's together, (and ensuring the same
vlans are on all ports between the 2810's
and all 2510's, if you are using multiple
vlans and 802.1q.) This should keep the
5 downlinks on the same 2810 in a blocked
state, and make debugging easier. (as it
is, I suspect random links will block,
and if 4 of your 5 2510's go down,
nothing will block)
Q4:
Stacking won't hurt, and is only useful
for management of the switches. It doesn't
provide any bandwidth or redundancy benefit.