can I extend license trials per user/key?
Yes, generate a trial extension and send it to the users.
can I Deactivate the original key the sample app used, then create a new one and use that? My guess is that this is acceptable test because in real life I would deactivate a user and grant them a new one - i.e. I have full control over the process
Yep.
In real life there aren't a lot of cases where you'd need to send a customer a new key. If they need to deactivate and they claim the original computer is no longer operable, you can just deactivate that particular machine within that license.