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.