Hey @Carlobentivegna
Thank you for your post and some great questions. Let me give some overview first, to clear some confusion ahead.
There is a clear difference with Sitejet Studio to Sitejet as an Integration with Plesk. So I can mostly answer the questions from the view of Sitejet Studio.
1.- Would it be possible (with an agency license) in Plesk to prevent a user from copying their website to another external Sitejet? And that the URL when editing “cms.sitehub.io” can be different on a personal domain?
When you have a Team or Agency plan with Sitejet Studio, you will have the possibility to set Sitejet up white labelled. For more info, take a look here: https://help.sitejet.io/hc/en-us/articles/24276052896919-Whitelabel-in-Sitejet
If you do not grant full access, the users can not export the website. I might have to check if they can use the copy and share feature. However, you should check this legally, because it might be that the design is your copyright unless you fully sell it to your client.
Plesk does not have the feature of the Project Manager, Customer Portal etc. About anything else, you might need to contact Plesk to discuss possibilities and ways to restrain the access.
2.- In the future, will Sitejet continue to be included in the cost of Plesk licenses or will there be a trial version, or will there be a need for licenses per site?
I am unable to answer this. You need to contact the Plesk support. Nothing of sorts is planned or discussed at this point - to my knowledge.
I ask these questions because my agency in the past used similar builders, which after several years of use ended up creating an additional cost for us whether you wanted to edit the website, after an initial design, or extra licenses.
We would like to bet on Sitejet completely clearing up these doubts.
I would really love to have you on board with us! But please take a deeper look at what you really need. While the Sitejet Builder is nearly identical in Studio and with Plesk, there are significant differences in Sitejet Studio - the Project Manager for example is designed for Agencies like yours to support and help you upscale and quickly expand. This might be something more useful for you - especially, when you need a whitelabel feature.
From my experience, it is awesome to have everything in and bet on one tool. But (!) no Website Builder or tool can fulfill all your needs and it would be a lie if we say we do of course. Please understand: Sitejet Studio is the All-In-One hosting and Website Builder solution. With Plesk you got the Sitejet Integration and have to run and maintain your own server. Also great, as you get access to the server files, if you need. And much more shenanigans. So both options are fantastic, depending on what you need and plan.
However, there will always be certain limitations and most of our heavy users and agencies do have a second Website Builder depending on the client’s needs. That is completely normal. 
Did you already sign up for free in Sitejet Studio? I highly recommend to take a look around and ask us more questions here or via mail with all that comes to your mind. If you need more, let me know.
