i wish to know what is the longest possible run that would be verified for celeste

according to any%

10 hours https://www.speedrun.com/celeste/runs/zqgorqxm

Runs can be as long as you want if you can upload a video of it to a video sharing site, but they do have to be a reasonable attempt at being fast. We don't reject people for just not being as experienced with the game and taking a long time to beat the game. However we reject runs for fooling around and trying to be slow on purpose like standing around for an hour. This does not include challenge runs as long as you try to beat the game fast with the restrictions placed upon you

how does one become a mod for celeste or any game on this site for that matter?

have experience with the game and the site