The mirror isn't infinitely powerful. That's not an unsatisfactory answer, it's an unsatisfactory attempt at FAI. Quirrel does say the mirror was never completed, or at least wasn't the final goal of those who built it.

Because the mirror has limits, it can only create one room (at a time); if it could create whole universes it would be absurdly powerful.

As for letting viewers interact with other viewers' creations, that sounds like a feature, not a bug. If the mirror was used to find out what the CEV of a viewer was, then it was useful for other researchers to observe it and take notes. A person viewing their own CEV would be a partial, unreliable reporter.