I cannot comment on the bandwidth aspect... but can on the the opto isolator problem.
I have had problems with optoisolators and the Ftx90 series. In some cases the device would not switch the unit to TX and or would die.
I found the very small DIL (TTL compatable, 14pin) relays the answer. If they do not have a overswing diodes built inside - fit one or two, maybe this is your problem with the solid devices?? you do not say what the failure mode is, the LED bit or the opto-transistor
I use the front Mic socket interface.