Done a bit of clicking,
Stage 1:
https://ridewithgps.com/routes/30382946The route from Plumpton to Penrith is the reverse of what "Lakes Velo" did on Good Friday for the Easter Arrow as a first leg down to Lytham Booths.
Penrith to EK is the obvious route; Looking at the satelite view there's a path alongside the route from EK through to Busby, there's no blue signs but "Whit is it no a core path, but?" should be enough to limit you to an admonishment if needed.
Stage 2:
https://ridewithgps.com/routes/30368002Erksine Bridge, the trace goes over on the left in both directions, not sure if there's only one track open or what, unlike the east coast bridges there doesn't appear to be a website for it.
I've made an assumption that Glen Coe will be hit in the late evening, hence just going straight up the A82; if it's a tad busy looking the old road through the village gets you off it for a bit, but it's a bit lumpier, the main road is actually ok gradient wise and you can hold a reasonable speed over most of it when heading East.
At Crianlarich I've put the trace out to the pub that claims to be open until 1, it therefore also passes the other 2 and then I've brought the route up past the shop, through the underpass at the station and onto the old Glasgow Road, because why not...
I've put the route down the cycle track from Arrochar along Loch Lomond; unless you're there before 9am it's just going to be carnage, bits like Pulpit Rock have been made easier to drive, but when half the drivers are in hire cars wondering how they're going to get through a gap between a rock wall and an artic. and the other half are screaming at the hire car in front that "You could get a bus through there" and then round the corner comes an 65 seater Trafalgar coach...
There is an argument for setting off early from Paisley, riding up Loch Lomond and turning off onto the A83 before the tourists have had breakfast and returning by the Faslane route if you're later in the day. Also remember that if you take pictures of Faslane you will automatically be assumed to be a Spy by the MOD particularly if you work for them.
I've altered the route through Alexandria and Renton, sod finding the cycle tracks, just ride faster if the Neds have run out of Dope and Bucky.
I'll do the return route later.