Well if you want to cycle between Dorking and Leatherhead then the cycle path is far nicer than mixing it on the dual carriageway (there is no parallel country lane option). I have wondered in the past whether something similar along the A264 between Horsham and Crawley would be feasible as similarly, there is no decent parallel country lane route, the main road in both cases has effectively hogged the best route between the towns.
BTW I am thinking about utility cycling here, not leisure cycling.
Dorking - Leatherhead I agree your choices are limited by geography but I always ditch the cycle path and go via Mickeham/Old London Road, when I cycle to Leatherhead from the 'sham which I have to do sometimes as there is a good indepenant SAAB dealer there. (though more often I go via Betchworth and Headley from here)
Alternatives to the A264 for utility/commutting? A lot depends on your definition of decent but I'd punt...
Forest Road via Colgate and Pease Pottage
Rusper Road via Rusper and Ifield (my recommended option)
Horsham/Crawley (partially traffic free) cycle route as shown
here
WSCC do not believe separate cycle infrastructure alongside the A264 is feasible on the grounds of cost.
My commute is the A281 southbound btw. Decent enough imo but plenty tell me I have a death wish.