Hi, susanh,
As the others have pointed out, your driving times are way too unrealistic. On day five, you need to give yourselves time to pick up your hire car, and I'm assuming that will be from Edinburgh Airport, which would be the most logical location. From there, you still need to get to the M9 for a few miles, then the M90 for Perth. From Perth, you'd take the A93 to Blairgowrie, then stay on the A93 for Braemar, then Ballater. The A93 between Blairgowrie and Braemar is not a high speed road! I would estimate at least four hours from EDI to Ballater, not including toilet and meal stops.
On day two, you mentioned driving or taking the bus to St Andrews, but you're not picking up the car until day five. It would be faster, and easier, to take the train to Leuchars Junction, then the bus from there to St Andrews. Unless you want to take the bus from Edinburgh which travels along the scenic coast of the East Neuk of Fife.
On day three, you may want to consider taking a taxi from the Royal Mile to the RY Britannia, then take a bus back from there. That would give you more time in both locations. If Holyrood Palace is closed when you're there, you may want to consider a walk up Arthur's Seat. Be sure to get a reasonably early start to take it all in, and check the closing time for the Britannia so you don't get caught out.
On day 6, it's going to take you way more than 1:46 to get from Ballater to Culloden. Where are you getting these times? The only way to get from Ballater to Culloden is to take the A939 to Nairn, then head west from there. You could conceivably take the A938 from Grantown to Carrbridge, then the A9 north to Inverness, but either way, it's a good 2 1/2 hours, possibly more. In late April, there is still the chance of snow between Corgarff and Tomintoul, so check the weather forecast before you leave Ballater.
Allow yourselves three hours from Culloden to Glencoe. It's a fair piece, no matter which route you take.
On day seven, it's going to be quite difficult to visit Mull and return to Glencoe on the same day. It is possible, but you would be so rushed that it would not do justice to Mull, which takes at least a full day, if not more, to enjoy. And you're in Scotland to enjoy yourselves! You'd have to leave Glencoe early in the morning, and in order to do Mull justice, you'd need to take a late ferry back, then drive from Oban to Glencoe in the dark. If you decide to take a boat cruise on Loch Lomond, Cruise Loch Lomond leaves from Tarbet, and Sweeney's leaves from Balloch and Luss. Both are an easy drive from Glencoe. You could possibly visit Mull, but you definitely cannot visit Mull and take a boat cruise on the same day.
On day four, after you visit Stirling Castle, you may want to consider visiting the Wallace Monument. It's a bit of a hike up the steps to the top of the monument, but the view from the top is worth it.
I just noticed that you have no plans for day eight (April 29th.). Was that a mistake, or is that an open day? If it is, you could easily visit Mull on that day, and even stay overnight there. On day nine, if your lodging in Glasgow does not provide parking, you may want to consider dropping the car off, and taking the express bus in to the city. Having a car is a liability in Glasgow.
I don't mean to put a damper on your plans, but there are a couple of days when you have way too much scheduled. The routes which you have planned are very scenic, however, and I'm sure that you will have a wonderful time in Scotland!
My very best wishes for your travels, and Happy Holidays!
Mike (Auchterless)