I actually have nothing more to add because this brilliantly says what I feel. I agree with everything you have written here!
I think people keep forgetting that Vividha did do all the vachans with Atharv before she got married to Ravish. Those were the vachans she believed in. The rest with Ravish, the mangal sutra, sindoor, are all just symbols for her. With her heart and soul she is married to Atharv.
And of course we will know from the flashback that she has already kept her fast for Atharv before because she expected to live her whole life with him. And just because he is not with her now, he is still in her every breath so how can she keep a fast for anyone else.
Great post!😳