If you bought and used these temples, you would also have to take over the graves?

There are temples that have graveyard and not.

Ryogonji's case, since there are only 4 danka(supporters), the ownership of the graveyeard will be transfered to Daitaiji (Nishiyama-san), because he is the chief prist monk for the both temples. It is also common for the "ward" to take care of the management of the graveyard if the number of the residents and danka (supporters) are relatively the same (which is not the case in Irokawa)