How to nicely tag footway where access is conditional?

If the tagging is correct, I wouldn't worry about it. There will always be edge cases were the rendering is not as we'd like; there will always be routers that don't understand foot:conditional.

At some point anyone writing a router or a renderer has to ask themselves how complicated they want to make it before they stop.

I tend to put the more "cautious" option in the standard tag and the other option in the conditional tag. Say there is a 30km/h speed limit in front of a school during day time I would tag max_speed=30 and maxspeed:conditional=50@("night time"). So an app not knowing conditional tags would display the safer 30 and a driver at night can take a joy in driving faster when they discover the signage if they like it.

Following that logic I would tag the private access as standard access and the permissive as conditional access just as you did it. That way hikers would discover a welcomed shortcut when they turn up at the location instead of having to take a long detour to what they had originally planned assuming a open path in March.

Still, I believe your tagging is a bit redundant. The tags
foot=private
foot:conditional=permissive@(Apr 15-Nov 30 dawn-dusk)
already define the rule completely. The foot= gives the standard case and the foot:conditional= the exception. Your additional ;private@(Dec 01-Apr 14;dusk-dawn) merely repeats the standard. There is no harm done doing it that way it may just cause tagging errors later on when someone else tries to update the tagging.