I was wondering if some Dunmer would still secretly worship the old tribunal? is it possible? and how do you think they would deal with the individuals that are found worshiping the old Tribunal?

It's quite likely some still do. After all, centuries of Tribunal rule didn't stop Dunmer from worshiping the Daedra and proclaiming all kinds of heresies about Nerevar, and the New Temple still honors the Tribunal as special saints. If you worship them in the old way (proclaiming their divinity), you will most likely be persecuted as a heretic, exiled, or locked away in some kind of Ministry of Truth-lookalike. The New Temple is, after all, not a new institution: it is the continuation of the old Temple with a new dogma. They're unlikely to change in that regard.