Yes, you create several Synology users with their own login data. You can assign and remove your own rights to each user. This can also be done in a "Friends" group. You can then give them access to the Surveillance Station and block the rest. You can already set this up and test it without the additional licences if you already have a Synology. The app for the cameras is "DS cam" from Synology.

You can choose this when you register. Perpetual makes sense

Yes. The licences can be used for all Synology NAS that have Surveillance Station. The licenses are therefore limited to the Surveillance Station software and not to a specific Synology NAS.

I bought the pack and you can't download it on the website, it's a credit card which contains an activation code for the licenses provided (i.e. 4 licenses) It is strongly advised to keep the card and the proof of purchase, because if your Synology breaks down and you are exchanged for a new one, you must take steps with Synology to deactivate the old registration and activate the registration on the replacement Synology. (this is my experience) If you order online today, Monday 24th December, you will be delivered on Thursday according to the website

Yes, this is possible. I will soon have to carry out this procedure myself. First remove the licences from one disk station (DS) and then add them to the new DS.