If you go to loch lomond As odd as it sounds id say stay away from the sign posted car park and visitors center.
In my eyes it's become a bit commercial, Where you actually park and at the visitors center theres a row of about 20 brand new shops and a seaworld like place that to me just seem out of place.
The loch itself is all locked off so to speak, you can only walk round about 500 meters of it from the visitors center.
You need to actually drive up to the north side and park somewhere off the beaten track to be able to get out and enjoy it, but luckily as your driving up past it to fort william though, that shouldn't be to big an issue. Theres also some other lovely places en route like Glen/Loch Etive and Glen Coe that id say are worth a stop just for a nice view.
I also kind of agree on skipping Glasgow, i live there thats reason enough, it's a nice place, full of lovely people (excluding me) but lets be honest it's not the prettiest of places, there are some lovely spots and some hidden gems and some good beer but again you need to know where your going and what to look for.
looking at your plan above, youve got alot of miles to cover and alot of driving to do and not much time, but you should see some bloody beautiful views and hopefully you might even get 1 day without rain