I tried a bunch of different things, and none work well. 😟 .
Here's what I would do to flatten a keyword list in Aperture. With only 200 keywords, it shouldn't take long at all.
Create a new top-level Keyword Group. (A Keyword Group is just a Keyword that has or will have children.) Name it "aa.MyBusiness" or something like that. (I preface all Keyword Groups with "aa." so they will stand apart and be listed at the top of the Keyword HUD and the Keyword Rule in the Filter HUD. Starting with the keyword beneath "aa.MyBusiness", select all the contiguous single keywords (in other words, select from the first keyword below aa.MyBusiness" to the last keyword above the next Keyword Group). Drag this set into "aa.MyBusiness". As needed, collapse the Keyword Group "aa.MyBusiness" by clicking the disclosure triangle. Next you want to expand the entire Keyword Group that is now just below "aa.MyBusiness". Do this by collapsing the Keyword Group (if it is expanded), and then hold down "{Option}" while clicking the disclosure triangle. This should expand every keyword group that is its descendant. Now select, in groups, all the contiguous solo keywords at any level and move them to "aa.MyBusiness". Repeat with the Keyword Groups once they have no children. Continue in this way until all your keywords are children of "aa.MyBusiness". Any time you asked if you want to merge keywords, say "Yes".
This should give you a single top-level Keyword Group, with all of keywords children of that one keyword.
From there, you can start to build a keyword hierarchy to meet your needs (or leave them a one "flat" list under "aa.MyBusiness").
HTH.
--Kirby.
