ISO Advice on Using Free Time before Heart of Scotland Tour

I went to Anstruther/Isle of May before a tour to see Puffins, but they will be gone by September.
I can strongly recommend using Stagecoach buses to get around if they work for your destinations. Easy to use, very comfortable, and no issues traveling with luggage.

You might consider this you tube video on Stirling. I found her whole series in planning my own trip to Scotland later this month. https://www.youtube.com/watch?v=rveHrt1dR9Q 48 hours in STIRLING, SCOTLAND! | Is it worth a visit?

You could certainly do the West Highland line service train, that is a walk up service. End in Fort William after going to Mallaig, then the bus to Inverness on the morning the tour starts.
In September you must be able to find somewhere at Galashiels in the Borders (on the rail line) for a couple of nights, use there as a base for touring.
Orkney and Shetland is a bit risky, for if the weather turns.
Lewis may be an option - fly from EDI, then either fly to INV or morning ferry to Ullapool, and bus to INV. Local car rental to maximise your time there. But, being Loganair, it's a lot of money to sink into such a short stay.
I'm sure you could find somewhere around Nethy Bridge/Grantown for 2 nights, for the Cairngorms- train to Aviemore, then local bus on its Aviemore to Inverness route.

Thank you for the replies! Will check the Stirling Youtube, Stagecoach buses (coaches, I guess), and, coincidentally, I became aware of Grantown that's on the Speyside Way only a few days ago.

As Monty Python fans, near Stirling we enjoyed our visit to Doune Castle. Also at Falkirk near Stirling, are the Kelpies and the Falkirk Wheel. Having a car made these easy to visit.

Stirling is a good choice. It's not far from Edinburgh and easily reached on the train. It will give time to recover from jet lag and there's plenty to do and see in the area.

Stirling Gatehouse Tourist Information Centre is possibly the best place to visit first to pick up maps and information about Stirling and you can also buy admission tickets here.

There are details of a heritage walk around Stirling here.

The castle is well worth visiting, along with the Church of the Holy Rude where Mary Queen of scots worshiped and James VI was christened. There is also the Old Town Jail or the Smith Art Gallery and Museum (free).

The ruined Cambuskenneth Abbey is just a short walk across the river. A bit further but doable by bus (or taxi) is the Wallace Monument.

Hello all, again I really appreciate you taking the time to reply. I've been studying the logistics and concluded that my time between arriving in Edinburgh on the morning of Sept 12 and needing to be at the tour hotel in Inverness on Sept 14 at 4pm is too short to allow for visiting Stirling. I was biting off more than I could do.

My revised and current plan is to spend Fri Sept 12 night in Edinburgh and get the train on Sat Sept 13 to Inverness. I'm thinking that I can rent a car on arrival to explore the Highlands and meet the group on Sun Sept 14 without worrying about logistics or time.

That's the plan as it stands now, ha! @laurie ann - the Stirling video was great. I hope you have a wonderful time, bon voyage.
@leonard - am glad to hear you enjoyed Doune Castle. By the way...What is the air-speed velocity of an unladen swallow?

Thanks again for your kind suggestions.