This is certainly a mystery. I'm getting the same results. Some ideas:
1) if you close Excel and re-open it you seem to release the lock on the database (not pretty but a workaround)
2) Use DAO instead of ADO (I've had good results with DAO before - but that's basically starting over, unfortunately).
3) Create a template DB and then you only need to copy the template DB file rather than building new a database from scratch.
4) Or continue researching this error - I found a number of sites where others report similar problems (no clear answer either - some folks give precisely these steps and report success, other report that it's not working for them).
5) Or use a default value so that the required field works (i.e., you won't need to change the field property).
I'm very curious as to how you got to creating databases in scratch from your original post (in the original thread) where you only needed to update a parameter value. This seems like an awfully long sidetrack and I don't see how this solves any problems for you (even if it did work). But of course it's up to you if you don't want to explain what you're up to - it would only be interesting to here what the plan is here (admittedly, I have my doubts - creating db's in code is generally more trouble than its worth and I would only do it as a necessity).