I replied earlier, but for some reason it's disappeared! Don't know if anyone else can see it?
Glasgow airport has daily direct flights to JFK and Newark, so may be of use to you. It may be no difference, but may be worth checking in case flights are cheaper or to save the one-way rental fee.
It might help us make recommendations if we knew more about your parents interests - For instance, someone said skip Glasgow - if your parents are into city stuff like museums/galleries/architecture etc then this isn't very good advice, but if they're very focused on countryside/scenery/highland castles etc then maybe it is (although in that case I perhaps wouldn't spend 3.5 days in Edinburgh either).
Here's my last post, sorry if repeating:
Day 5 - It's a short train journey from Edinburgh to Glasgow, but nevertheless by the time they check out their Edinburgh hotel, get the train through etc, they'll not have much time to see much in Glasgow - The city is markedly larger than Edinburgh and the attractions are more spread out so I'd add a little more time there. Even if they took the train through from Edinburgh in the late afternoon or evening of day 4, that would at least give them the whole of day 5 in Glasgow.
Day 6 - You don't mention where they're picking the car up from, but I'd strongly recommend Glasgow airport. Picking up in the city could be quite stressful with the one-ways and bus lanes etc, whilst the airport is on the right side of the city for heading to Oban - you exit right onto the freeway that takes you across the Erskine bridge and onto the road to Oban and the Highlands. You can get from the city to the airport by cab (about £20) or the airport bus (https://www.firstgroup.com/greater-glasgow/routes-and-maps/glasgow-airport-express).
Days 10-12- 3 nights isn't enough to see Skye and Harris imo. I'd either stick to just Skye or add at least a day more.
Days 13-14 - Inverness may be called a "city", but its really a town of 60k people (making it about 1/10th the size of Edinburgh and 1/20th the size of Glasgow). As such, there's not actually much to see in Inverness and I wouldn't recommend spending 2 nights there personally, 1 is enough at most imo. They could even skip Inverness and drive straight to Pitlochry and stop to see Culloden on the way - although admittedly this would be a long drive for one day.
Day 15-16 - Again, Stirling isnt that big and there's not that much beyond the castle and Wallace monument imo, so its probably not a 2 nighter. Id maybe do 1 night Pitlochry and 1 in Stirling the night before flying home.
Day 17 - Dont know if flights are booked yet, but Glasgow airport is a similar distance from Stirling as Edinburgh. If the car's picked up from Glasgow they could save the one-way rental fee by handing the car back there and flying home from there - GLA airport is a major airport too with direct US flights.