Yeah, the GB magic item list doesn't cover everything by a long stretch, so don't go by that. There should be 2 potions in the place you got the heal scroll (unless we're talking about 2 different places).
Spoiler alert:
It's in the bookshelf at the top of the stairs on the second floor of the Church of St Cuthbert
If you have a modded version, I guess it's possible they removed it for whatever reason.
As for use magic device, you shouldn't need that. If that's how you're trying to use the scroll, it's no wonder you're failing. Give it to your cleric (make sure he's got enough wisdom to cast it) and then use it from his inventory on the radial menu (backpack icon). If you're a spellcaster with the spell on your class list, you don't need use magic device at all.
And if you can't find the potion, you can always use the console to generate a new one.
I've never ended the game that way, but I'm sure it's possible. Funny.
