Well, no. The only difference is the temporal 'location' of the observer. From the perspective of the Enterprise-C crew, the reality they visited was 'a possible future' in the same way than the future Q showed to Picard in AGT was. Doesn't mean either of them wasn't 'real'.
Not at all. The future the Enterprise-C visited was created by a very specific thing not happening. Thats the entire point of the episode. The ship got swept into a time displacement vortex and wasn't there to fill its role. Therefore, an alternate timeline was created in that moment. Just like Neto traveling to th 2250s via Red Matter Black Hole. Just like McCoy saving Edith Keeler.
The stuff Picard experiences via Q's screwing around is just some random extrapolation of one of any infinite amount of possibilities. By definition, that makes it meaningless. It's one of any infinite number of outcomes that may or may not happen