I think you're spreading yourself too thin for what will basically be a week on the trot.
Aviemore to Ullapool is a good four hour drive. There is no ferry at 4:00 p.m. Ferries Monday to Friday depart Ullapool at 10:30 a.m. and 5:30 p.m. If you take the 5:30, you won't get in to Stornoway until 8:00 p.m. Unless you're travelling in June, it's going to be dusk by the time you drive off the ferry. The Saturday ferry departs Ullapool at 7:00 p.m., and the Sunday ferry leaves at 6:30 p.m.
Unless you leave Aviemore at 5:00 a.m., you're going to have to take the late afternoon ferry. Don't forget that you have to be at this particular ferry slip 45 minutes ahead of departure, otherwise you may lose your slot to standby cars.
Given that the ferries from Tarbert to Uig depart at different hours each day, you may have as little as 12 hours on the islands. And that includes sleeping time and driving from Stornoway to Tarbert. That's not nearly enough time to fully appreciate Lewis and Harris. Unless you have a bucket list to fulfill, your time would be better spent with an extra day on Skye, or a day on Mull.
Days six through nine are very doable. Days three and four are the ones you need to reconsider. If you truly want to visit the Outer Hebrides, you could eliminate staying overnight on Skye, spend day five on Harris, then take the Tarbert to Uig ferry on day six, drive straight across Skye, take the Armadale to Mallaig ferry at the other end, and drive on to Glencoe from there. It would make for a long day, but your total driving time would only be about five hours. Doing that, though, you'd completely miss all that Skye has to offer. Again, you'd have to be very aware of the ferry schedule. There is only one ferry on Tuesdays and Thursdays from Tarbert to Uig, and on Monday, Wednesday, and Friday, if you miss the 7:20 a.m., you're not going to get to Skye in time to catch the last ferry to Mallaig. You could always take the Skye bridge, but that will add lots of drive time, and you probably wouldn't reach Glencoe until late evening.
I hope that this information is of some help to you in formulating your plans.