@Austin Healy Fro the first question, based on my personal understanding, the answer is yes. When we change the primary user of the device, The "Enrolled by" property of the device is not changed. So I think it will not lower the device limit count.
To check the count of a DEM account, as a workaround, we can order all devices in intune by parameter "Enrolled by user UPN" then check the count. Or we can try the script in the following link :
Note: non-Microsoft link, just for the reference.
In addition, applying an Azure AD device restriction to a DEM account will prevent you from reaching the 1,000 device limit that the DEM account can enroll. Please confirm if there's any Azure AD device restriction configured.
Hope it can help.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.