Removing demand/economic factors, how theoretically
feasable would it to finally build the unstarted (if that's the right word)
Garve - Ullapool line plus extend the Kyle of Lochalsh line to the Isle of Skye itself reaching Portree and/or Uig?
'feasibility' depends on cost, so you can't really remove the economic & financial cases from such a question. And that will never justify this line under current appraisal criteria.
In terms of engineering practicalities, assuming you're largely following the road, you have 3 major challenges: the first is a general one, in that to get a route without excessive curves you'll have to go a bit of a way up the side of the hill (see the West Highland Line) and that will mean bridging every side valley. If you stay lower you'll have to deal with all the watercourses. The other 2 are finding routes alongside the Lochs, given the roads run very close to the shore, and the Corrieshalloch gorge. You're looking at cutting into the hill a significant distance above the roads in each case. The line will also be terrible for buildability, with very remote, inhospitable terrain and the need to bring in a workforce from far away to build the line.
The only saving grace of the line is that it would score highly on reducing economic isolation for rural communities, and it can be single line with maybe 1 passing place, using the existing RETB (modified slightly for standard point machines).