I think the reasons are -
After the wedding Ram and Priya will have to give lot of shots together for long time so Rma may be adjusting his dates to finish his job in films
Also .. we read that Ram needs some time to get over his eye problem .. so perhaps they created episode bank and let him go for treatment and come back
Shots taken earlier when merged with episode shots will have some disconnection since due to improvisation the new shots could be different
Also ... there are lot of actors and they may have other engagements .. to shot groups acts such as Mehndi, Sangeet, Wedding lot of people have to be handled therefore it is not good to keep main characters also occupied
Hats off to actors, they give good performance / emotion even with doubles
ALL SAID AND DONE -
CHEAT IN MANNER SO DON':T GET CAUGHT
Audience is very smart so pay attention to details.
Puhleeezzz