Stephen, if you get to Inverness then the hard bit's over as far as high ground / low cloud base is concerned. However you are correct to have a few alternative routes up your sleeve just in case.
After Inverness route north to BONBY staying above 2000' for R610D. Then up the coast taking care not to stray into D703. You also need to be above 2000' for R610C. DACS is available for both via Tain range if you cant make the 2000'.
Turn north abeam Lybster and then route via Loch Watten pretty much direct track to Kirkwall.
Perth - Inverness tracking the A9 is possible with a 3000' cloud base. The only tight bit is Drumochter Pass to the north of Blair Atholl.