I know if marriage happens when Bidaai will end (i hope minimum 2 yrs😊) Alekshi will end up together.Even if sadhana look alike will come it does'nt matter as she is not sadhana .In the beginning Alekh may believe her to be sadhana ,but as he gets to know her ,he will realise that she can't be sadhana & he will know how good sakshi is (if she changes to Good😉)
If Alekh gets normal after marriage due to another shock (most likely scenerio) then I don't see how a Sadhana's look-alike would fit into the picture. Another thing if Mamaji does her kanyadaan then its Alekh and Sakshi forever.