Keep Coding

If I backup and restore with the Google Datastore backup utility, numerical IDs are copied to the new Datastore.
The restoration "manually" assigns numerical IDs -- i.e., it is not using the auto-allocation.
Then, if I create new Entities using the default ID auto-allocation, it seems that I risk collisions. As stated here:
[A]dvanced applications may sometimes wish to assign their own numeric
IDs manually to the entities they create. Be aware, however, that
there is nothing to prevent Datastore from assigning one of your
manual numeric IDs to another entity.
How do I avoid such collisions? I'd rather not switch to using names, to maintain compatibility with my existing id-based code.

If you restore into the same project, it is just reassigning the existing IDs, not manually assigning 'new' Ids. This this case, you will not run into any issues with auto-allocation. As a related note, we track auto-assigned Ids per entity-group, so you cannot copy child entities from one group to another.
This works across projects due to internal APIs we can call.