For the last year, Celebrity has not been granting permission for early disembarkation. However in the last month or so there are a couple of reports where the permission has been granted in Europe. So it is certainly worth asking but you would need to already have the booking and then ask. It sounds like you want to know if you’re going to get the permission before you book and that probably isn’t possible.
To get the permission, you need to file a form and pay a fee which I think is still $65 per person. It will be checked to make sure that there is no violation of any US law and that the government of Mexico will allow it.