I need a script to create a user, given username / password / email ; the script should check if the user is already existing, and in this case update email and password

There is no way to update the password via command line because that could be a huge security issue, but you can create users from command line if you need
to:

mystic -newuser handle=g00r00 email=me@mysticbbs.com pass=1234

It will return "TRUE" or "FALSE" to STDIO depending on whether or not the user is created. It will not update an existing user, it will return FALSE if the ID is already taken.

Do you have a way to detect in your script if the user is new or existing?

I suppose I could do a "update" function that allows a user to be updated as long as their security level is less than 250. I don't know that I want to create a single function that either creates OR updates a user though, I'd
have to think that through more.

--- Mystic BBS v1.12 A47 2020/09/25 (Windows/64)
* Origin: Sector 7 | Mystic WHQ (1:129/215)

something like this may do the job:
mystic -newuser handle=g00r00 email=me@mysticbbs.com pass=1234 -updateifexists

Okay I will experiment with adding an "update=true" option for you. But as with the creation it will only work on users that are less than security 250.

or may I do something like this with mystic pascal ? (I have tried
python, but did not manage to have it work)

Yes this could be done in MPL too but hold off on that I will try to give
you a better way to do things. I should have time to work on that today.

--- Mystic BBS v1.12 A47 2020/09/27 (Windows/64)
* Origin: Sector 7 | Mystic WHQ (1:129/215)