Okay here, I am adding my own twist to Jyoti's theory!
Jyoti, exactly as you have said in Theory No. 1, this Dhruv may take Kaajjal to a cliff side and get angry with her for her dhoka and Dev's harkat etc, ... for which then Kaju may explain all the background as you have said about herself and Dev and all, so this fatso will finally get to know the truth ... but Kaju will also tell him that since Dev now does not want to marry her she is going to commit suicide! She may run to the edge of the cliff and threaten ... Dhruv will be shocked to hear this but he will also be trying hard to stop Kaju from taking this ultimate step ... he wonders what to do ...
Meanwhile again as you have said, this Uncle will frantically call Dev that Kajal is missing ... and in fact they both may be extremely fearful now that Kajal will end her life because not only is Uncle against but she feels no hope because even Dev is against! So now Dev will promise to somehow find her and save her ...
...and in a fit of compassion Uncle may say "Dev, if you find her please marry her, because I would rather have her alive for at least a short time even if she has to die after marriage, rather than to allow her to die today without her wishes being fulfilled!" Dev too will think this is the best because he too does not want Kaju to die in sadness without her love being fulfilled!
Meanwhile while Dev is frantically searching for Kaju, he will not even know where to look for her as there is no trace of her or any clue ... but suddenly he may get a call on his mobile - from Dhruv! At first he may not want to answer but then he reluctantly will answer ... and this Dhruv (who by this time has got full truth from kajal!) will tell Dev that he finally knows the whole story and he is very sorry for what happened ... Dev will think its just an apology phone call and will try to disconnect because he is anxious to talk all this later and find Kaju first...
But Dhruv will interrupt and say "Listen, I have Kajal here, I brought her here in anger and she told me everything", and Dev will be relieved to hear that! But Dhruv will continue and say "But she is now run to the edge of the cliff and is planning to commit suicide, I don't know how to stop her ... I am continuing to converse with her to distract her, but you have to rush here and stop her. I cannot keep her talking for long!"
Dev will then dash into the car and drive like Michael Schumacher in a Ferrari ... and reach the cliff just as Dhruv is almost given up hope of saving Kajal! Together they will pull Kaju back and Dev will say "OK I will marry you" and then the two will hug each other! Dhruv meanwhile sees all this - and feels remorse that he never really noticed how much these two are in love and how he thought he was in love but his love is nothing compared to what these two have!
He then decides to somehow help Dev-Kaju's love because he feels guilty and responsible for separating them! He then decides to go with Karan to Sid baba and somehow get some upaay for saving Kaju's life for Dev's sake!
Maybe Dhruv will find the upaay too ... because remember Dhruv has promised NT once long before that he will help Dev have a good life again! So maybe it will be Dhruv who will actually help find that missing upaay that can help Dev-Kaju live happily ever after!