1.How much time will it take to get the entire issue resolved? (Best case and worst case scenario and the time lines for each step of the case, like for legal notice and then other steps)
This is a practical issue. The time taken for each process will depend on the prevailing circumstances and the place/state. Nobody can predict the time taken even an approximate time because it depends on various factors which you will come to know when the issues are being dealt practically.
2.Is there any way in which we can find a faster solution to getting our money back as we being a start up, the amount being stuck right now is quite significant?
Since it involves issues locally, you may have to look for such event in the local.
3.How much does it cost approx. for the entire case in the event of the case being dragged for some time?
The advocate fees and the court fee will depend on the advocate your may engage and the state which you belong, the court fee differs from one state to another.
4.What are the chances of us not receiving the money in the event of the vendor using a loop hole to prevent payment.
No predictions hence no answer.
5.We are ready to issue a legal notice to him and is there any way we can make the case more air tight and force him into negotiate as he always cares about his name in the market.
You can ask your lawyer to do it the way you desire, he may be having more answers to such questions.