Here in lies the problem. Your boot fitters are fucking morons! Now that we've established that lets move on.
When it comes to boot flex a 7 to you could be a 3 to me and vice versa there is no defined method to establish what boot flex is. I ride the stiffest boots K2 makes and I find them too soft after they break in, but they fit so good that I just deal with it.
Now lets look at some ways to remedy this situation.
Take your Insano's and back over them with your car this could potentially break them in or destroy them. Either way it's a win win situation I think.
Second solution rip out the stock insole get an aftermarket off the shelf or a full custom. I recommend full custom as it sounds you need to build up the heel pocket to compensate for a smaller ankle. Next pull out the liner and with that new footbed in there look at where the ankle placement is, mark it with a grease pen, sharpie, blood of a virgin and get some J Bars, C Bars, Butterfly wrap, or some thicker 3m foam. My guess is with your ankle is that you'll need either c bars on top of a butterfly wrap or to cut an O ring then with a j bar on the bottom to suck up some volume.
After putting those on the boot and blasting them quick with a heat gun to help them adhere to the liner put the liner back in the boot. Tighten the boot back up and stand flat footed. From there flex your knee over your toe with all your force and see if there is any lift/aggitation. If yes we proceed, if no get drunk!
Now that we've established that you're still having issues it's time to pull that liner out again and stare at it. Give it a mean glare like you're Clint Eastwood about to shoot it dead on main street. OK now that that's out of the way you'll want either a tongue shim or some more of that 3M Foam. I prefer tongue shims, they're white and easier to cut to the proper shape. Grab one of those and set it in on the outside of the liners tongue (note this won't work with a burrito wrap liner). Duct tape it in place then put the liner back in the boot and tie it up again. Proceed with the knee over the toe flat footed shit. Does it work? Yes, proceed to get drunk and fist fuck a hooker. No? OK, time to move on to more customization.
Take the tongue shim out again, grab your trusty table mounted belt sander and safety glasses. You may also need a razor knife to trim to fit. Don't worry about cutting these things apart it's all good. Trim it to fit the tongue. If it looks like this is the proper thickness and where you need it cut, adhere it to the tongue of the liner put it back in the boot. Did this fix it? If yes continue to flog that farm animal if no turn on the belt sander and put the glasses on. While the sander is going to town put the tongue shim on the belt and sand off a few mm's and remember to bevel the edges, the reason for the beveling is to make the shell tongue wrap around the liner better.
After you do this put it back in the boot. Tie it up. Did this solve the problem? Yes, Cool go shred and come back and fuck 2 Norwegian bikini models. No? OK now we're going to get even more advanced in the boot fitting.
Pull that liner out again, give it a mean stare, again! Good got that out of your system? Now grab some more of that 3M Foam and some duct tape! Build a foam back spine up the back of the boot that looks like a T so that it goes from the base of the heel then branches out around your overly tiny girl calves. Adhere it to the liner, then cover it with duct tape to prevent it from coming off. Put the liner back in the boot, again! Tie it up flex around and shit see if that fixes the problem.
If it fixes the problem send me a personal check for 200 dollars. If it doesn't it's time to check out some more boots.
Now I personally recommend something like the K2 Maysis or T1 DB or UFO. It's a single shell boa which if you know how to flex into a boa boot you can get it tight around the ankle and calf. The internal boa harness is adjustable to the shape of your instep and that helps with ankle issues. It also acts as a double shelled boa without all the bullshit you're dealing. I know your pain as I was forced into the Thraxis (triple boa) due to a boa malfunction of epic proportions.
If none of this works quit snowboarding and take up sit skiing. You'll be the recumbent biker of snowboarding.