Tobirama mistreated the clan and discriminated against them. Individual relationships with particular Uchiha's doesn't even excuse his actions against the entire clan.He did, He gave his life so his whole team could escape, which included kagami.
He also stated in the post i showed that kagami wasn't the only one, He said there were many others like him that earned his trust.
Your just pulling at straws here.
He could simply lie to the uchiha and swear the others to secrecy, If the uchiha were so discriminated by all the village as you claim why would they squeal on their beloved hokage for some mere uchiha?.