Elizabeth, it sounds like your feet are statistically out of the norm, so it is understood why you have difficulty getting proper fit. Now, with that in mind, which way is it that your left heel moves? Up and down, or left to right, or front to back?
It seems like there are two obvious ways of padding the problem, and those are by padding the shoe or by padding the foot. You are doing both, I guess. Does a super thick sock help? If that fouls up the fit in front too much, then make yourself some custom socks by sewing together the back of thick socks with the front of thin socks.
If you get the right thickness of padding into the shoes, you ought to be able to sew it into place in order to get it to be somewhat more permanent. I don't know those particular shoes, but the other Inov-8 shoes seem adaptable to that.
Once you get as close on that as you can, then you might be able to toughen the foot skin so that it doesn't blister easily. There are several ways of doing that.
–B.G.–