In order for the locations to be published
in Google Search and Map, they will have to be verified first. Please check
this document for more details.
For ads themselves, unverified status doesn't prevent ads to use that location as its extension.
In other words, yes, the ads continue to run location extensions after migration.
For question #1,
The auto-migration will always create a new GMB account.
Of course, this would result in duplicate locations in some cases, but that's fine.
As mentioned above, unverified locations can be used as location extensions so this will not affect ads serving.
(You can think about unverified locations as phone book entries--anyone can create the same address entry in his/her accounts)
Chris, please let me know if the above still doesn't address your question. :-)
Cheers,
Thanet, AdWords API Team