Under A, the spell was targeted at everyone in the room rather than the room. After casting, it stays with the people rather than the room.
Dispelling a spell does not break part of a spell, it breaks the whole spell. This can actually be used for some fiendishly cleaver traps.
The person is "carrying" the spell on them. It is the same spell as affects the other people who were in the room. So if you use magic to manipulate or dispell it then it affects all of it. However it is not actually an AC.
Bonus, under B the only real part of this that changes is to dispell you now want to target the room since the spell is there rather than on the individuals.
Also a spell is Target: Individual when you are dispelling. The actual size of the spell does not matter. So a spell that is the size of a bottle of water (say creates a ball of light) is the same for targeting as a spell the size of a city (say wraps the whole city in darkness).
I don't think the answer is that clearcut when talking about multiple targets. From the corebook p.157:
The Individual target for a Vim spell can refer to either an individual spell, or an individual object or person, and similarly for Group.
And then later:
Spells and magical effects do not have sizes, so size modifications do not apply to the levels of Individual Target Vim spells. However, Vim spells affecting areas, or number of spells, must be increased in level for large areas or large numbers, as normal.
I'm inclined to say that generally, if you have a spell that is effectively targeting several individual targets instead of an area (such as A described above) you have one spell that caused several effects, and you need to dispel each effect separately (note that you usually dispel an "magical effect", not "a spell").
For this I would say that under A the spell is not in the room anymore, so no, and no because of the reasons I stated above.
I think it's also not clearcut that you can have an AC to a particular spell. I'm not saying you can't. I just don't recall any specific case either in favor or against it in the rules.