Followup.
Assuming Roles access is set-up properly, which I do not want to dive into...
1) Solved. Create table with schema API call from script, than, next call is used to populate it. Table dropped in finally{} scope afterwards.
2) Solved. Fell back to calling stored procedures, with internal PREPARE, EXECUTE, and DEALLOCATE PREPARE, parametrized by table names. Calls made from script, using API as well.
As a result - just one GET call to scripted PHP service, passing comparison data packet, returned are 'updated' and 'new' packets, stuffed into response content.
Bonus: I'm two days wiser.
