You should rethink to the way you're presenting the material. Anyway, here's a working solution.
The colon problem occurs if the prependcaption option is set in the package options.
If you remove this option, the set caption (or the colon, if the caption is empty) is no longer displayed on the ...
I take it that the OP is not actually interested in why there is no \bottomsep macro, but rather in how to achieve the spacing issue they face.
As the OP states correctly, \topsep + \parskip is added ...