Would my suggested change have any downsides?
You already listed two downsides. I can't think of any more. The key
thing is if the advantages are worth the disadvantages, and that's a
tricky one to decide...
Would this be an accurate summary of your suggestion: Bold can only
start immediately after whitespace, or an apostrophe? I think that's
equivalent to what you said, but I'm not sure. If it is the same, then
it might be a slighter easier way to describe it to users.