Itch has never revealed any information about its inner workings, so my answer is based only on my assessment as a user of Itch and this forum for a few years.

What I have seen is that although the staff is willing to help, it seems to be a very small staff for the volume of people they have to serve, and possibly for that reason, they will never answer you unless necessary and sometimes the time to resolve a dispute can be long.

Normally, they review the information you send them and if you actually prove that the game is using your material without the corresponding permissions, the game will be deleted. This could be tomorrow or within a couple of weeks, depending on how busy the staff is.

The best thing would be to have a lawyer advise you on how to prove your point, but using normal channels. If your lawyer contacts them to threaten them, the most logical thing would be for them to refer the matter to their lawyer and that will only result in much longer delays.

My personal recommendation is that you first talk to the project leader, indicating that if he removes you from the list of contributors, but continues to use your work, you will have to report him.

Also, you can contact the organizers of the JAM, with some exceptions, jams are usually less than 100 people, so it will be faster for the organizer to intercede or even remove the game from the JAM.

And if you do not get anything by talking to the project leader, then contact Itch support, and make sure to put all the relevant background and evidence, so that when they review your ticket, they can resolve it without delay.

PS: When you contact support, you should receive an automatic response with a ticket number. If the case takes too long, you can try to go to Discord with that number to request that they expedite the case.