I like the idea of making it essentially two separate contests running concurrently, since the layout would be locked in before we start and the gamemode inherently allows mix-and-match.
The problem is that in the 2nd part people will just go for 1 layout and mostly ignore others unless they made it themselve.
Thats why the 2ss contest only had 7 maps. All the ones who showed to be less good were often just ignored. And of the 2 winners intercept was obviously too hard for most to detail on because of the entity work in it (and its sensitivity on breaking).
Focussing on 1 map for detailing is just better.
The detailing maps wouldn't be populated or playtested, and the popfiles would use the unaltered source map.
They must be playtested with the #1 popfile. That popfile has proven to work so if the map breaks anything it will be shown fast.
Idealy you just pick 1 map and set that one as the target. And better would be if that map has a certain reward aswel. The valve one for example was ensured to be in the game which is a good motivator. But still, mapping should be done because its fun anyway, so to me the medal was enough (with mvm_intercept i went for 1 before)
Also, i made a quick mockup shape of an mvm map that i wouldnt mind giving for the artpass contest (note that this was just 1h of work):