Wing it – apart from Fort William. A recent tour of the north and west, I did intend staying in Lochcarron, and don't know what possessed me to decide on heading for Fort William and spending the night in a B&B there instead. Once place told me they had a room, but as it was a double, she couldn't let me have it because she'd be losing money (as opposed to leaving it empty?) and another which was at the top of a flight of steps just hadn't bothered putting the No Vacancies sign out – which might not have bothered me had I not been wearing heavy bike leathers on the hottest day of the year so far.
Also, Applecross is tricky for accomodation. TBH I wouldn't bother with the campsite there, unless you want the facilities, just set up on the north end of the beach. I mention this because, even in a car, the road to Applecross (the Bealach na Ba) is just awesome, and the scoff in the Applecross Inn is excellent.
Durness (as far North and West as you can go) might be difficult as well, but if you absolutely HAVE to have a bed, just keep going, you'll find one. Remember, it's the far North and in summer you'll find that when the day turns into the gloaming (which is ideal for roaming), night doesn't really happen and you're unlikely to be dealing with driving in darkness.
Treat it all as an adventure and you can't go wrong.