Wonderful analysis, Khyaatee..đ
You know what, it's like you wrote down my thoughts! I was thinking exactly the same thing..Khushi, her sis, & I were having a discussion about the whole thing last night..And we were certain that the person who stopped Gauri from entering the garage/store room (whatever it may be) is Nikhila for she had long, straight hair and ever since Dadaji & Dadi's wedding, she's been sporting that look..Also, the change in her look suggests that there may be a revelation of her other side...She's always seemed too good to be true..
When she saw Gauri at Shyla's room, she must've understood that she may be searching for clues leading to Shashank...
And yes, as you said, once the truth comes out, the track would revolve on Shiv & Gauri's relationship...Personally, I don't think she'd leave him, esp. if she learns that she could save his life...It would be then that Shiv would realize the depth of her love and would protect her...