Isn't it fun to start figuring all this stuff out?!
I'm struggling in the same kind of snow, but am getting the hang of it more and more thanks to how many storms we've had this season. For me, it's letting the skis "run" down the hill just a millisecond longer, with just a wee bit of tipping. Voila! They will slice when you do that.
I'm so glad you love those Sheevas! Funny, I think I'm going to sell mine, because I don't care for how they ski in untracked powder! I'm sure it's a balance issue and how I initiate turns in powder, but regardless, untracked is something I can find on many days here even after a storm, and I need my daily drivers to handle it comfortably for me.
As for the extra space from the heel lifts--Skisailor's idea of duct tape on the boot sole is good, or you might go see a fitter and ask about adding foam to the liners in that area. It could come down to being lifted a bit higher in the boot (as she also alluded to) or taking the volume up from the top, which will keep you where you are in relation to the cuff.
One thing I've learned from SO (we bicker about stuff quite a bit, he's very VERY opinionated, as one would expect from someone who has been in the biz full time for 30 years) but at times, I think his opinions don't work for me. Well, one thing he has said over and over is, "if you put padding inside the boot, you need to put it equally on both sides, or it WILL throw you out of alignment/balance inside the boot." So, if you pad around the ankle, pad around both sides. If you pad over the instep, pad over both sides, if you pad the cuff, pad both sides, etc. I am finding he is pretty spot-on with this. So, if you pad the instep area/top of the ankle, make sure it's even across both sides.
It does sound like the heel lifts were causing more issues than fixing. It's nice to have the quads tell us when we are in the back seat, and be able to fix it, vs. being stuck there no matter what--I know that feeling too well!!