Hi again, losrichins,
Stuart (isn31c) has given you some great advice regarding getting out of Glasgow (well, Paisley!). Just to add a wee bit to that, the road that you turn left on to from Murray Street is the A726, Greenock Road. The Erskine Bridge is not very far from the airport. Once you're on the bridge, stay to the left to make getting on to the A82 somewhat easier. Ask at the Arnold Clark rental counter if you have any questions/doubts.
The Arnold Clark office opens at 8:00 a.m., so there may be a line of potential renters waiting when you get there. Best to get there about 15 minutes early, and have all of your papers in hand. Perhaps Stuart can answer this - How easy is it to get a taxi from Gilmour Street station? Would it be easier to take the 500 bus to the airport and take a readily available taxi from there? Also, losrichins, does Arnold Clark offer customer pickup to take you to their office? And would that pickup be available before their opening hour of 8:00 a.m?
For the Tobermory to Kilchoan ferry, you should be able to purchase your ticket starting tomorrow (February 18th). That will guarantee your spot on the ferry, but you still need to get to the ferry slip about 15 minutes ahead so that you don't get bumped. If for some reason the ferry is cancelled (weather, breakdown, etc.), you can hustle down to Fishnish for the frequently operating ferry to Lochaline. In that case, drive to Salen (mainland, not Mull) and head north from there.
Next question - Yes, reservations are pretty much required for dinner, especially in some of the more isolated and tourist oriented areas. Anywhere on Islay, Mull, or Skye, and definitely Plockton. Although all of those places have decent takeaways, if you don't mind eating outdoors, in your car, or at your lodging. Hint: Bring along some plastic cutlery, as it's not always provided by takeaways. You can always dispose of it/recycle it at the end of your holiday.
If you enjoy hiking, there are two very good hikes out of Mallaig, although you could just walk up to the end of East Bay for good views of the harbour. You may want to consider visiting Camusdarrach Beach, which is just off of the B8008, just east of Mallaig. Camusdarrach Beach was used as Ben Knox's beach in Local Hero. It's one of the prettiest beaches in that part of Scotland. You can continue on the B8008 to Arisaig, and for more adventure, you could take a day trip on the MV Shearwater, which departs from the Arisaig marina. And, as previously recommended by Stuart and others, you could take a cruise with Western Isles out of Mallaig, either to Knoydart/Tarbet, or on one of their seal watching cruises.
In answer to your Plockton to Newtonmore question, which way you travel is entirely up to you. If you are pressed for time, by all means go by way of Spean Bridge. If time is not a factor, I'd recommend going by way of Achnasheen. First of all, the drive to Lochcarron from Plockton is lovely. Then the drive along Glen Carron to Achnasheen, and the A832 and A835 will dump you out on to the A9 and a relatively easy drive through Inverness. By then, you'll have plenty of driving experience, so Inverness will be a piece of cake! If you had even more time, you could take the A896 from Lochcarron, drive through Glen Torridon, and pick up the A832 at Kinlochewe. Now that would be a lovely drive!
With all of this information, you are going to feel as though you've already been there before you even get started!
Best wishes once again,
Mike (Auchterless)