I've not found out about premium connectivity yet - I've only had the car since June.
In my case, the lease company were given my work email address and used that to add me as the secondary account, so I use that to sign in with.
Service appointments are a little bit grey. Tesla want to deal with me, via the app, and I'm not sure how much the lease company know/get told, as the owner, via emails, etc. I'm pretty sure they wouldn't bother managing their Tesla fleet via the app.
My experience is that the lease company don't/didn't know how to deal with Tesla. The ordering and delivery process wasn't particularly smooth. Rather than try and solve any problems, the lease company just blamed Tesla. This left a sour taste to be honest.
Day to day, it's fine. The app works as it should, supercharging works fine, my own Spotify works fine.
Luke