Re: CQL scaling limit?
If you know how to handle MetricManager, that sounds fine. I was thinking in more basic terms: adding some log statements to you indexer java code.
Regarding the id block allocation, some features seem to have been added, which are still largely undocumented, see:
Notice that the default value for ids.authority.conflict-avoidance-mode is NONE. Given the rigor you show in your attempts, trying other values seems worth a try too!
Best wishes, Marc