To sort out the

issue with the bonus crystals, here is a list of all levels that have crystals that spawn after the level begins, and that can be reached with an umbrella. There aren't very many. If you grab a

on the level before one of these, or a

two levels before it, be sure to let us know whether it lets you skip to the crystal spawn, and I'll update this list with a YES or NO.
Keep in mind that 0.5 is the only one that is truly the end of the PD, so any of the others will probably just act normally, and this most likely answers our question. Nonetheless, I will track any new info we find.
Triggered by the end of the level:
Boss levels ~ NO (impassible)
0.5 (last) ~ YES
3.3 (sort-of last) ~ NO
13.2 (middle)
26.3 (second-last)
Triggered by Igor:
86
0.4 (second-last)
11.2 (middle)
72.1 (second)