Scotland in 9 Days! (I"d prefer 7)

You would be pushed to do the itineray in the time scale using public transport.

Day 3 - West Coast Motors (who get good reviews) do a day trip from Ona to Iona. However you could book this yourself with ferry from Oban to Craignure, drive to Fionnphort for the foot ferry across to Iona. This would give you the possibility of using the ferry from Fishnish to Lochaline and then driving onto Fort William for the night rather than going back to Oban. BUT this would make it a very long day!

Eilean Donan Castle is a bit of an outlier here. Is this an absolute must see? (OK, I know it is the iconic image that appears on all the packets of shortbread, but the outside is much more impressive than the inside.,. The castle was actually rebuilt from a ruin about 100 years ago so isn't as old as it looks.)

You could cut out a day of your itineray by combining Days 4 and 5 and omitting Eilean Donan castle. From Oban drive via Fort William to Culloden and then spending the night at say Grantown on Spey. You may be lucky and see Highland cows at Culloden - see this thread....

You could then combine Days 6 and 7 by driving down through the Cairngorms to Edinburgh, Avoid the A9 and take the much more scenic 'Snow Road' - details here.

This does become an intensive itinerary but woukld satisfy husband's preference for 7 days...

This is probably the tour you need on Day 3. https://www.staffatours.com/tours/three-isles-early-bird-tour/ It does bring you back to Oban. It’s a long day with an early start. However, I don’t believe the puffins are in residence in September (double check this), so a different tour or an overnight on Mull might suit better.

Yes the puffins in Scotland, not just Staffa, depart back to sea in early August so it is very unlikely you will see any in September, it would be a real bonus if you did.

Being realistic looking at your projected itinerary for 7 days it will be a whirlwind tour plus add on the stress of using only trains and bus lines will make it more hectic than it should be. I say use all three if possible when most feasible and PRACTICAL. Let me share with you my Scotland honeymoon trip to give you an idea.

1. Book a place to stay at Portree on Isle of Skye. Rent a car of your choice to Isle of Skye. But know there are dedicated bus lines to Portree. Drive to the Isle of sky and stay at your preferred Airbnb/hotel. Enjoy the local sites and eateries.
   1. Wake up early the next day. If you got there by bus get on a local bus from Portree or drive your rental to THE OLD MAN OF STORR. For me it was a relatively mild hike with majestic scenery. Doesn’t take long. Seeing nature’s own cathedral towers are one of a kind. If you still have energy go to the QUIRIANG and hike along the higher or lower trails. It’s a decent walk with a splendid view. With the Quiraing you can decide how far you want to go. You will find a fair amount of folks at either locales but more foot traffic at the Old Man. I was able to do both places in one day easy.
   2. Next day make it a sight seeing trip. Take a drive south and visit Torabhaig Distillery and after that drive to tour Talisker Distillery. Both shouldn’t take long then head back to Portree for the last night. OR.. Skip the distilleries and drive early to see Eileen Donna castle before reaching Fort William for a day. Next day from Fort William go to hike at Ben Nevis. It can be a doozy for city slickers but the views are breathtaking. Coming back down get a bite to eat at the Ben Nevis Inn and Bunk House..WOW! Great food.
   3. Next day drive to Glencoe and stay at Strath Lodge Glencoe. From there plan your hiking treks. For me that’s a two day stay. Glencoe is magnificent in Scottish grandeur. It’s a great end to your Scottish adventure until the next time you visit.
   4. This last part is optional. My wife and I took a train from Glasgow to Penrith in the Lake District for three days where we stayed at the lovely THORNYTHWAITE FARM. A quiet and lovely Airbnb. From the farm we walked (yes, walked) to various scenic places along the way (Aira Force Waterfalls) but most notable was taking a small ferry boat across Ullswater lake to Glenridding for one of the best gourmet burgers that side of the world. No joke. It was ridiculously good. Then we walked back around the lake to burn off the calories. You can also visit Thirlmere lake and stay at the King’s Head Inn or Derwentwater and stay at Ashness Farm. Both very scenic and rejuvenating. The cool thing about this itinerary is all the places mentioned are relatively close and on the west side of Scotland and England with Glasgow to Penrith being the longer commute. Driving from one location to the next is beautiful in itself. With the train ride being just okay. Hope this gives you ideas. Best of fun!

As for the early chunk of your itinerary, Stirling Castle is definitely doable by public transport from Glasgow; I've done it myself - though be aware that the walk up from the train station to the castle is steep (but totally doable). As for getting to Oban, it's very much the other side of the country, and if doing it by train you'll have to change in Glasgow as that's the only way to get to Oban from Stirling. It looks like a roughly 4-hour train journey and they're not at all frequent, so do factor that in!