I've done a few flax fiber backings, but still need to experiment with it more. I don't know the answer to all your questions, but I will tell you what I do know.
I haven't had a thin layer of it overpower a belly yet. Maple is the softest wood I've used it on, and it was fine. It is very strong though. I once had a v-splice glue line fail underneath a thin layer of flax, and nothing catastrophic happened. I didn't even notice it for awhile. The bow held together and could still be shot. (But obviously after I noticed I took it apart to fix it)
I know even a thin layer does some work because, if you reflex the bow during the glue up it will hold some of the reflex. That said, aside from reflexing I've never noticed much performance increase from a flax backing either. Could be I just haven't used it with the right woods on the right design yet.
I imagine it would work well with ipe. But keep in mind that hard backings like hickory and bamboo tend to be much thicker than soft backings, and that will make a big difference. You will need a whole lot of flax and a whole lot of glue to make it as thick as a hard backing. I've never put flax on that thick myself, but I would worry that the weight of the glue would make it not worth it.
Yes, it is protective to the back, partially because its inelasticity prevents the back of the bow from stretching too much, making splinters less likely in the first place.
And yes I see no reason why you couldn't pre-make backing strips. No idea how they would perform compared to fiberglass, but if they were similar that would be really neat.