The documentation that came with the collection of "interchangeable" buttons indicates they aren't meant to be removed once placed, however, it also indicates the use of a thin, flat blade should allow for their removal. While the third "main" button isn't mentioned in the documentation, it appears to be the same as the two "interchangeable" buttons so I wonder if it might also be removed, if done carefully?

When you use the term "stock extra buttons", I'm wondering to what you're referring? Blank buttons? If so, do you suggest simply using blank buttons instead of labeled ones or do you know of a way to have them engraved?