The ID is not unique AFAIK. For example when restoring a database without cleaning first could leave you in a world of hurt because id's would already been used and unique violations would appear. This makes me rather sure that the ID's are not unique over multiple instances.
I'd go for the UUID option.
No it is not unique. It is a unique number constructed out of the entity type (first part) and an id (second part). So, it is unique per app, not over apps. See my more detailed answer on this questionfrom 6 years ago: https://forum.mendix.com/link/space/databases/questions/87488
Here I explain the real details on how Mendix created the GUID.