I was playing around with custom league settings and it seems quite flexible. You can choose the length of the regular season and the number/length of playoff rounds, among many other things. With 12 teams you can have 2 or 3 divisions for example, and set up playoff byes/wildcards accordingly.
I would suggest to keep it simple though and not get too crazy, but if the preference is to not use week 17/18 then we would have to either make playoff matches 1 week long (might be too harsh), or shorten the regular season. For example with 12 teams you can probably make it 11 weeks and play each team once. There are also double-header options but that seems a bit weird (you play two teams at the same time). Maybe it would work on non-bye weeks. We'll have the long and boring off-season to figure this out.