Will composite indexes or unique indexes help with query performance?
(t2.`owner` = 'ABC123' or t2.`user` = 'ABC123') 


This search term will force a table-scan. Composite indexes won't help.

Suppose you had a composite index on (owner, user). It could do a quick lookup for owner = 'ABC123' but then the desired values for user could be anywhere in the table, not just in the subset matching the desired owner value. So it has to scan the whole table anyway.

Likewise if you had a composite index on (user, owner). It could do a quick lookup for user = 'ABC123' but the same problem exists, finding the rows for owner will force a table-scan.

The one remaining possibility is to use index merge union optimization, but in my experience this doesn't work as well as you'd think.