Things are weird. Dadi mentioned 'R didnt get Krishna in dwapar yug, and seems like R wont get M in this yug'. So she very well expected things to happen even if he was engaged to D. M was surprised, rather shocked, that she knew of R's 'prem'. Wouldve been good if he questioned her 'wisdom' in letting R stay there if she knew about it. But I would guess D/K wouldve done the needful which wouldve made M defensive. Dadi seems to be a smartass who knew what she was doing all along.
Yeah R wont try to kill people so she can be considered good i suppose and Dadi may just want a good future for her gd (she has always been the one who poked the love angle towards M - something what a friend or sometimes a random stranger in movies/shows does).
Cant blame D for what she is doing now - the forced confession seems to have given R the freedom to hug M willingly. D is the one who smelled something fishy even before any of the audience sensed anything. But she has been pushing R towards M and vice versa, indirectly all along.
Mohan - he couldve been cautious I'd say.. but knowing 'i dont have anything about R in mind' like he said before, does give him the freedom to do and say things which though it comes from a genuine place can be misinterpreted. Some scenes pushed it a bit too far in that respect.
Promises... well... they matter only if it is required for the plot. (Why is the ghost even required if she cant take care of D?)