regarding (b), the rating of the speakers really wood depend on the capacity of the amp to deliver current. my electrocompaniet amps, for example, put out serious current & are stable down to at least 0.5 ohm. so, if bridged, they'd still be stable down to at least 1 ohm, & wood still drive most any speaker.
re: your question regarding horizontal bi-amping, assuming identical amps, no difference in gain should be needed - the speakers' built-in x-over wood be seeing the same amp for all intents. but, i guess ewe could experiment... as far as needing active x-overs, again, ya won't need 'em w/identical amps. but while it sounds illogical to me, ral sez ya mite get even better sound if ya *do* use 'em - at least he did w/his maggies...
re: (c), this *is* where having a gain control on at least one of the amps wood be useful - different amps may have different gain characteristics, especially if ya wanna, say, run toobs on the top & solid-state on the bottom.
hope this helps, doug s.