You've put yourself into an impossible box. Unless all of the space before and space after are a multiple of 15 pts you cannot lock in the value and hope that the text will always end aligned to the grid at the bottom. You could try not aligning the bullets to the grid, and that would result in some variation in the space after, but it still won't work if you have a bullet at the bottom of the column.
I think that there are too many contradictional requirements. In general it is impossible to justify vertically and align all paragraphs to the grid and to have different spaces between paragraphs. Too many parameters.
It remembers me, a client wanted: Make the font bigger, make more space between lines, let more white space on the border of the pages, add some text and reduce the number of pages. Not everything is possible.
I figured as much. Just wanted to make sure I wasn't missing something. I told them that to maintain alignment it was not really possible especially when the bullets wrapped to another column. Just trying to manage expectations and to insure I wasn't crazy.