I'm not going to repeat what the others have said, but I completely agree with their points. Do not communicate your desperation. Nothing good can come of that.
I got phones from my other ex-colleagues (from the same company)
saying that they heard rumors that I am joining back. After hearing
this, I became little worried as these guys might create some
impression about me on interview panel, that might cause interview
process to become biased. Shall I inform about it to John?
You're not thinking clearly. Who do you think told those ex-colleagues about you? It was obviously John.
Besides, put yourself in John's shoes, a little bias can be a very good thing. If you hire someone, hiring a former colleague reduces a lot of the risks because you already know how well that person works and how well that person gets along with others.
In other words, John probably removed himself from the selection process because he wanted to maintain the appearance of objectivity, not because he was trying to create an atmosphere of perfect objectivity (which would be impossible and possibly counterproductive anyway).
I really do not want to miss this opportunity. How shall I convey
message to John that I will work very hard for him and with complete
dedication. I just want to make him feel to hire me anyhow. But I do
not know how to convey that message to him.
Please don't! The more desperate you appear, the less likely you are to receive a job offer. This is human nature. The fact that you do not know this worries me.
And I know it's been said already by others, but let me say it again here. Do keep sending out resumes and do keep interviewing with other companies. You must do this for practical reasons.
But you must also do so to make yourself feel more confident. The more interviews you have (even if they're just in the pipeline). The more confident you will be during your interview with this one company you really want to work with.
And finally, the next time you apply for a position anywhere, check with LinkedIn first (there is even a browser extension you can install). And if you know a friendly former colleague who works at a company you want to apply for, ask that person first. Even if that person just tells you to apply through their HR portal, the very fact that you contacted him/her first can mean that he/she can put in a good word for you with HR.
When I used to work for HR (a long time ago), we even had a special checkbox in our database for resumes like that. The idea was that if someone internal referred someone to us, they would probably ask us about them again, so under no circumstance, could we lose that resume, nor could we screen out the candidate right away, otherwise our department could get in trouble. We also made sure that such resumes would be on top of the pile of resumes when a hiring manager looked at them (so they wouldn't be lost in the shuffle).
And in addition to that, since you're a software developer, it's also very possible that if someone refers you internally and that you get hired because of that referral, that the person who referred you will get a nice bonus for having referred you.