I have a lot of options now
One plan is seats to Fort William, walk out (Ben Nevis is not going to be the target for my first hill/mountain walking visit to Scotland for a long time) somewhere but avoiding the mountain tops, stay in a bothy, walk on to Corrour, seats back to Birmingham. Looks like around 30 miles in total, carrying a fair amount, so that plan might change.
An alternative is walk to Glen Nevis YH and spend the second day walking as far up Ben Nevis as I feel I need to.
Either way, I
might sleep on the way back despite being in the seats.
Or explore Fort William for a bit, train to Mallaig, stay on the coast, come back down on Sunday (shame about the 1939-1950 connection from Mallaig, which feels a little risky, especially as NationalRail seems to be insisting that the CS leaves at 1900).
Or stay somewhere, Ember down to somewhere, jump back on the CS halfway back to Glasgow.