I think vehicle allocations are more fluid than we think, I do not know if the ex South E300s are permanent or just a temporary measure.
It would seem more single deckers are promised from Scotland.
I think the 'No Merc' was a 'nice to have' rather than a physical necessity, someone may know otherwise??
As others have said, Double deckers are problematic because Peasmarsh has limited maintenance facilities and going back in time, the London & South Western Railway, when it built the main line from @ Surbiton down to Basingstoke, built most of the bridges over roads to a lower height than would be acceptable today, Aldershot & District had a 'Lowbridge' decker fleet and latterly Alder Valley had some low height VRTs & then Olympians.
A few years back the 'Victoria Arch' otherwise known as the main north south road under the railway in the centre of Woking had its height limit reduced (note the limit was reduced, not specifically the physical bridge or road).
Double deckers can physically work the 91, just a little hard to travel there from Guildford.
The newly extended route 1 to Merrow Bushy Hill uses some 'deckers so reintroducing them to Merrow.
The 65, despite being officially converted to single deck has Tridents most days, as does the 'Kite' from Aldershot.
The 53/63 did use 'deckers some years back, no low bridges but limited passenger demand and probably a lack of tree lopping rules them out. I think someone on this forum pointed out that one of the PT school routes still serves Cranleigh using a Trident.
I remember a couple of years ago when Leas Rd still had a couple of Volvo B7TL 'deckers, they used to schedule one on the 18, the route with a typical passenger count of 3 each way?
It occurred to me today that with the takeover of Arriva, Stagecoach has almost re-created Aldershot & District in terms of area covered - Woking - Guildford- Cranleigh - Haslemere - Farnham - Aldershot - Fleet - Farnborough - Camberley - Bagshot - Knaphill. Only the routes on the east side of Guildford - 1(part) 6, 715 are in the former LCBS area.