I concur with ACraven about the weather and, more to the point, the long drive. If you can fly London-Edinburgh or London-Glasgow for the Scotland leg of your trip, you'll be much better off.
Not being hikers, we didn't go to Glencoe, but we liked Inveraray so much on our first trip that we made a point of returning the next year. The castle is really lovely, inside and out (including the gardens), complete with the duke's flock of black sheep. I believe you can hike on the castle's property up to the watch tower above the town, though we didn't try this. South of town toward Furnace, the Auchindrain Township open-air museum is absolutely fascinating if you have any interest in the history of everyday people, and it has miles of walking trails. There's also horseback riding at Argyll Adventure. Within a half hour's drive out of Inveraray toward Oban /Connel Bridge you can see the scenic ruin of Kilchurn Castle and stop in at the Ben Cruachan "Hollow Mountain" hydropower plant visitor centre (which sounds nerdy but is actually really fascinating).
Inveraray is a charming small town, all designed by one architect during the Georgian period. I can recommend Brambles for lodging and daytime meals, and the George Hotel for dinner and pub. There are several shops in town selling traditional Scottish woolens (but do read the labels as some are made in other countries).
Of course (not to be a wet blanket, but realistically), your entire plan depends on the virus situation by September, which is very hard to predict.
