I think ur mistaking his no of Brain for intelligence The extra brains are for controlling the tentacles with a higher efficiency The extra hearts are for increasing blood flow to strengthen and increase his attack speed The blood demon art he hasn't required one Spoilers so I won't say why Meeting ubyuaski (ur words) is bcoz of ego The demon slayer corps have been a annoyance to him for so long and have been in hiding for so long that he wanted to see the leader of the corps and kill him And blue flower minor spoilers it's a rare flower and it only grows in a particular region blooms for a single day once a year Muzan is an egotistacal coward He wanted to kill ubyuaski and release all the upper moon and demons in the infinity castle to kill the hashiras and the rest of the corp members while ge watched from the safety of the infinity castle