The major problem is that from Inverness the line goes to and terminates at Kyle of Lochalsh and then you have to connect to Mallaig.
The traditional way of doing that was two fold-
either a ferry on the incredibly scenic coastal run or the bus across Skye.
The ferry is gone 20 or more years ago (I had the huge privilege of doing it in it's last years). However I have done that far more recently on a wonderful little country house style cruise ship- the itineraries vary, but can include Mull/Iona/Oban/Fort William/Inverness via the Caledonian Canal- a wonderful, wonderful trip but very expensive. I still get the brochures, if it wasn't so expensive I would do it again in a heartbeat maybe one of the small isles trips this time. One of those things where if I was told I had a few weeks or months to live one of two trips I would book instantly.
The bus from Kyle to Mallaig can be done, but it is no longer direct. You would change at either Broadford or Portree.
I would highly suggest taking the Citylink bus from Kyle to Portree and have two nights in Portree, hang the expense. On the day of arrival take the afternoon bus to Dunvegan and back.
The next full day take the Trotternish Peninsula circular bus.
Another scenic way to do it is to take the Kyle of Lochalsh to Elgol bus at 1150 off the train, the bus turns at Elgol back to Broadford, then Citylink Broadford to Portree.
The Trotternish and Dunvegan bus trips can be done in a day with a break in between at Portree.
That way Mum has got her dream of Skye in as well.
Mallaig to Oban is a really tough choice on how to do it- either Mallaig to Crianlarich over Rannoch Moor, change. train to Oban; or Mallaig to Fort William, Ember bus or West Coast Motors bus to Oban.
Both are such superlatively good routes.
What I wonder is to take the bus route to Oban, then afterwards the train route back to Fort William with the Crianlarich change and then the Ember Bus or Citylink bus back to Inverness.
Sadly you will no longer find good deals (if you mean cheap) at Mallaig- the big hotel at Mallaig had a fire a few years ago, and put it's prices up hugely afterwards. There are two hostel options at Mallaig, but clearly not appropriate for your Mum.
Another really good option (and better priced) would be to take the Mallaig to Fort William train as far as Glenfinnan. Then either stay at the Glenfinnan Hotel, or the former Sleeping Car at Glenfinnan station.
I don't know if you are aware that there are two rail routes from Edinburgh to Inverness- the direct Highland line and the indirect route via Aberdeen (where you change trains). Of course you could add a night (or two) at Aberdeen (or Stonehaven) to include Dunnotar Castle. Or the wonderful new Ember Bus route Aberdeen to Dundee via Balmoral and Glenshee.
