1) Yes, only one hw-offloaded bridge per switch chip: that means that on devices with one switch chip you can have hw-offload only on one bridge, on devices like RB4011/RB3011 - total of two hw-offloaded bridges, one for each port group, and on devices without the switch chip - no hw-offload obviously.
2) Nothing specific here. If you are afraid of possible loops - run STP or RSTP on bridges.
3) One hotspot server will be enough
You are welcome!