as for the suspicion, james clearly didnt suspect wormtail,nor did he suspect sirius. so that implies he suspected lupin.
No, I never got that. James and Sirius were the best friends in the Marauders, so for that reason, James trusted Sirius. However, he didn't want to have the secret-keeper be someone that would be an obvious choice, so instead of choosing someone James was particularly close to, or someone who was very competant, James chose Peter, knowing that no one would suspect him of even being the secret-keeper. It was actually more a compliment to Sirius and Lupin that they weren't the ones James went with, since he wasn't going with the people everyone knew he would probably go with.
and one more query, snape has to be an accomplished occlumens to spy against Voldy, then how could Wormtail survive in the presence of Dumbledore so long?he doesnt look like he could study occlumency, even if Voldy taught him.
I think you're asking why didn't Dumbledore just go through the ranks and read the minds of everyone to ferret out the one leaking information to Voldemort...
I don't think anyone would have even suspected Peter of being the one going to Voldemort for a second. He was a little talentless nobody who worshipped the ground James and Sirius walked on. That's what he had always been. I think he was just kind of overlooked in the suspicion because nobody thought he had it in him to go to the Dark side.