I do not have experience with Scotland, but I do have a caveat.... Airbnb almost always what us millennials would call "outdated" locations left at the minimum if not plenty of perfectly updated ones as well. I find that there are almost none that are rated less than a 4 of 5, but many are very utilitarian and lots of private rooms in people's homes tend to remain unbooked unless there is an absolutely monumental event. (Olympics, a convention, a major eclipse viewing, etc...)
For instance, if you wish to Stay in Durness tonight (their time), there are 27 open options. In Rogie Falls, 44. Many look quite lovely and charming and you will have a lot of flexibility with a car at your disposal. I highly doubt vacancies will drop to zero in the fall. You can sort by instant book and message the host to ensure they are up for letting you in that day or the next. Many may require two nights... but many are also affordable and I can't imagine a host would mind you paying for two nights and staying one if you are desperate to move on. People pay for a railpass to afford them freedom all the time. If you have the budget, paying for two nights once or twice when you want one seems like a relatively small compromise for the opportunity to do as you please.
I think for the style of travel you are suggesting, staying in private rooms in homes (some that offer breakfast) might work quite well. You don't have to risk having trouble checking in and hosts often have great insights about where to eat and lots more.
One more suggestion: Shop through airbnb in locations you might want to stay. Save the places that interest you or that you think may remain unbooked to your wishlist... you can even make one for each stop. That way, you can split the difference between wasting time on the road or not. You will have a few options ready to roll. Worst case scenario, they're booked and you have to do a quick search or drive to the next town.
I don't know if Italy is any indication, but often apartments over restaurants and in family businesses never make it to airbnb or booking.com. You can always ask travelers or pub keepers at one stop about places to stay at the next stop and give those places a call then use airbnb as your fallback.