Well, I'm pretty certain, that "spell list" there means the "spells known" of the spontaneous caster.
I'm also very sure, that you can only cast such a spell once, and it then disappears from the ring. It's not a contradiction, the first part just means, that you have to expend a spell slot, so you cannot cast the spell, if you do not have an appropriate slot ready (all used up already). The second part then goes on to say, that the spell disappears from the ring afterwards.
For now, I think it works like this:
The spontaneous arcane spellcaster has to wear the ring to use it.
Whoever casts spells into the ring, does not have to wear it. (debatable)
Any spellcaster of any tradition can cast spells into the ring. (debatable)
Only spontaneous arcane spellcasters can cast spells out of the ring.
If the spell is on the class spell list, the wearer can cast it, using his or her own slots.
It doesn't matter, whether the original spell was arcane or divine. (debatable)
After a single such casting, the spell disappears from the ring.
Prepared casters, on the other hand, have to store the spells themselves, others cannot provide them. (debatable)
The big question here is... do they have to cast (expend) the spells to store in the ring, or do they simply decide, which spells are in there during their daily preparation?
Nifty item, but it could really need a better description.
Bye
Thanee