Rajbir maynot be fake - he did mention in one of the episodes that he was interested in modelling and acting for which he came down to Mumbai and tried a lot but returned back to Pathankot when his dad required his support in business and he took over from his dad..
Maybe from the exposure he received from Perfect Bride where he was one of the most loved kunwars, prodution houses might have noticed him and gave him a chance. There is no point in saying no when an oppurtunity you long craved, comes knocking on your door. I think this need not have any direct effect on his relationship with Priyanka or his plans to marry her. Don't actors/models fall in love and get married??? Maybe this is also his chance to be financially independent as there was a mention in the show that he fears that his family may disown him if he goes ahead and marries Pri without their consent and blessings..
I think we shouldn't jump to conclusions just because he appeared in another show unless ofcourse, there is concrete proof that he was not genuine..