It's a little odd that the whole premise is own your data and then when you go to order and there's a recurring subscription fee that you can't opt out of
I guess if the data is on your helm on your home, and you can access it from your home after cutting your internet, then you do own what's on your helm.
Yes, and you can. All services are available on the LAN on standard ports, even if the Helm server is not connected to the internet.
Plex is kind of like this, though Plex Pass isn't integral to the base functionality and you can buy a lifetime membership at least.