Guys, if they say the flight to ABZ is booked, I love the way you're all ignoring that and saying 'fly to Glasgow'!
Perhaps you might accept as a given for whatever reason they are into ABZ? Given they are flying EMA-ABZ, that's a low-cost so not changeable (Flybe?)
I looked at the map and thought 'Oh they are coming for a week' and then saw three days.
Like most other visitors to the forum, you are being very ambitious in terms of what you can see and do in 3 days. If all you really want to see of Scotland is the view from the windscreen, sure, go ahead and do this. You won't actually have time to do much (like visiting those castles you are driving past) but you'll certainly be able to say you've circumnavigated Scotland.
Other thoughts - you'll want to go down the west side of Loch Ness (the A road) not the east side (the B or C road which will take a lot longer to drive for no obvious gain).
In later September, you can probably wing it with B&Bs to some extent - but do check out your route for any areas where you may be dependent upon just one or two (i.e. pinch points). Also the better ones will book up more quickly. But especially if you are mid-week, taking a chance should be fine.
To answer some other questions
4. Speaking of photographs, I'm assuming that I could get good photos of the night sky, but are there any locations that you'd recommend above all?
No, most of the NW highlands have no light pollution so once you start north from Loch Lomond, the night skies will be open. Does depend on there being no clouds, which is actually the bigger issue. This is Scotland - land of the cloud and rain. The light on the west coast is different from that found anywhere else (except bizarrely East Lothian) so do take the time from relentless driving to actually enjoy that.
5. Are the lakes closed after a certain time?
From memory Loch Ness is about 30 miles long, and Loch Tay about 15 miles long, with communities living up and down the side of them. It is difficult to close a very large geographical feature which people live on and work on and live by
BTW, in Scotland, we call them lochs, not lakes.
6. Is it safe to roam around at night? (Had to ask. No offence meant)
Generally yes, but midges are worse at night. That said, they should be dying down by then. If they are a problem buy Avon Skinsosoft. It doesn't stop them landing on you (which means the annoyance factor is still there) but it does stop them biting.
7. How are the driving conditions? Also, do all the places in the list have some sort of parking arrangements?
Expect to average 30-40 mph on everything that isn't dual carriageway, and you'll be realistic. It is out of season, so the roads will be quieter, nevertheless, you are generally restricted to the speed of the slowest vehicle you encounter - which is usually a guy in a beat up Peugeot or Micra doing 40.
Roads you've identified are variable - from the good quality dual carriage way A90/A9 between Aberdeen and Stirling to really quite narrow A roads in the Highlands and Morayshire (for example the Elgin-Glenfiddich road I drove last week, and that particular section I was lucky to get to 50 mph on - it was just to twisty to get any speed on. Oh, and I was stuck behind a guy in a Peugeot anyway on the way south again
).
Much as I love Loch Tay, I'd have to question going there. It's not a great road, and while the area is beautiful it might be better to push on and spend more time not driving at one of your other places.