I tried it out (I have been taking a factorio break) and the special limits don't quite work. It seems to double-count them when finding the sum of limits and it doesn't flag it as "correct" when you reach N-1 trains.
Ex: I have 12 Plastic trains, and 13 parking spaces of non-limit train stations, and 1 dynamic station that currently has a limit of zero.
With the option turned off, the UI shows 12 trains with 13 spaces, but also has a warning saying there are dynamic limits and it does not flag the schedule as valid
When I turn the option on, now it says there are 10 total parking spaces, and it doesn't warn that there are too many trains.
I didn't see any obvious clues about why this happens in your mod code. It's puzzling. Maybe I should just wait for the expansion to come out...