The ankle closing is a good question. I think you need some range of motion before you hit the wall of a concrete boot. McPhail says one should have 15 deg before resistance in order to activate the stretch reflex that helps balance, protects your knee etc. So locking the lower leg, (and foot/ ankle too) is not a good thing. In fact, likely quite bad. I suspect one acl I blew years ago in literally a split second was from an overly locked up shin/lower leg.
I suspect a lot of the problem of stiff boots is they're just not set up properly and/or the feet/ankle or lower leg is locked up some how. Even having a cuff too tall could be a serious problem. Couple that with not enough forward lean and they'll never be able to "get over" the front of the boot. Like being in a hole.
Mushy boots one can get away with a bad setup.
I've done some work on my own boots ala McPhail: flat foot bed, no orthotic, 2 deg delta, toes spread, room to pronate and evert, top buckle loose so shank can get into stretch reflex and not lock up the ankle. I'm into it. I'm not a fan of the "wall" though. I use a softer boot tongue (120 flex on Dalbellop cabrio boot ), with a booster strap.
I'm loving it so far, been skiing this setup for over a year now. I can obtain higher edge angles while carving. I'm getting the automatic foot evert and rotate around the ankle as I rock from heel to first metatarsal during transition.