Hey Simon! Thanks for reaching out.
A) Totally possible, but I think there are other options worth exploring first, especially because those tongues are next to impossible to find.
B) 100% support this decision. Touring boots don't necessarily need to be super stiff in the forward flex to perform well, but the extra height will help a ton. Hole patterns might be a bit different though so you may need to punch a hole through the power strap to get it to fit. Either way, good idea.
C) Definitely a good idea. Play is never awesome.
If possible, a burlier liner with a stiffer tongue will make a substantial difference in downhill performance. Keep in mind that by adding extra downhill performance you will very likely wear out the boot even quicker because it's getting skied so hard.