Wow, that's quite a route!
A couple years ago I drove up to the Isle of Skye from Edinburgh and I found the Scottish Highlands to be absolutely beautiful. While I can't comment on all your questions, I can give the following advice based on my own experience:
1. You'll spend more time than you anticipate i.e. a driving route you expect to take 5 hours can easily turn into 8-10 hours. Driving distances take longer to cover in the Highlands and you'll definitely spend a lot of time frequently stopping here and there to take pictures. There are also many roads with single lanes so be patient and drive carefully especially at night when it gets foggy.
2. There are lots of rest and pullover areas but I recommend having ample food supply on hand and having an emergency gas canister/container in the trunk that's fully filled (in case you become low on fuel and can't find any gas stations for miles and miles).
3. Expect wet and windy conditions. It won't be pouring the whole day but from experience it will drizzle a little then become clear/sunny again and the same thing repeats a couple times a day.
4. I recommend reserving a B&B ahead of time since September is still considered peak tourist season in Scotland.
5. The best food in the Highlands are found in local pubs. Almost all will serve locally sourced produce, meats, seafood, etc. This is especially true the more north you get. If you want something really Scottish, try Haggis (a dish made from sheep organs) with neeps (turnips) and tatties (potatoes).
6. For photography, the higher north you get the better the photos!
Enjoy the Highlands!
