रामायणे विश्वासटिप्पन्यः

SVs don’t have a problem with Rama or Krishna worshipping any deity. Krishna says in Mbh that Vishnu’s head bows down only to Vishnu & it is taken by the Vaishnavas to mean that when He appears to be worshipping other deities, He is worshipping the antaryamin i.e. Himself. Rama has worshipped several devas in the VR and SVs don’t deny it.

Now coming back to rameshvara episode, the issue is different.

One - there is no reference to Rama worshipping Shiva to cleanse brahmahatya in VR. Ravana is called as lokakantaka as well as a man-eater and to claim that killing him begets brahmahatya flies against logic.
Two - one cannot do praayashchitta for the sin before even committing it. And VR records that Rama worshipped samudra before crossing the seas.