In Terms of the Topology and Smartscape API, would I be correct in saying any process group name, listed with "...../api/v1/entity/infrastructure/process‐groups" and doesn't start with "PROCESS_GROUP-" is probably a manually configured group?
No not really. All process groups have a discoveredName which is what Dynatrace detects on its own. This can be overridden with a DisplayName.
All Process groups have an entityId which always starts with PROCESS_GROUP-.
What do you want to know? how to understand process groups that depend on custom detection rules?