Anyway, I'm not entirely sure about all the situations you've presented to us, but these are the songs I'd give to the four of them.
Ragini: 'Deewani Mastani', because of the line "Jo jag ne na mani, toh maine bhi tani, kahan thi main dekho, kahan chali aayi". She used to be so angelic, but then the world turned against her love and she changed into someone who is prepared to go to any lengths to get what she wants.
Laksh: 'Tu Hain Meri Kiran', but just swap 'Kiran' with 'Swara'. I mean, Laksh has pretty much admitted that "Tu haan kar, ya naa kar, tu hain meri Swara", so what other song suits him better in regards to Swara? 😕
Swara: Of course as it has been mentioned before, 'Ek Hazaron Mein, Meri Behna Hain' suits her well. And really, Ragini is indeed one in a thousand, after all, not all sisters are so wonderfully kind as her. *eye roll*
When hopefully Swara will start to love Sanskaar (but perhaps hasn't realised it), then the song for her would be 'Na Jaane Mere Dil Ko Kya Hogaya'. 😃
Sanskaar: I would have originally said 'Tadap Tadap Ke Iss Dil' or 'Sun Raha Hain Na Tu' when he confessed his love but Swara broke their friendship, however now, I'm not entirely sure.
I think 'Dheere Dheere Aap Mere Dil Ke Mehman Hogaye' fits him in relation to Swara. I'd also say 'Tujh Mein Rab Dikhta Hain' and 'Muskurane Ke Wajah Tum Ho' suit too.
SwaSan: 'Hum Aapke Hain Kaun', because why are these two fasting for each other if they're not giving their marriage a chance? 😉
A general Sanskaar vs Ragini song: 'Aa Dekhe Zara, Kis Mein Kitna Hain Dum'.