So, you won't get 3 cases and a stroller in a Giulietta without dropping one side of the rear seats.

To be honest, you might not get them in a regular C-Max without doing so, either. The Grand C-Max would work as if you dropped the 3rd row of seats, there is plenty of luggage space, and there is also masses in the 500L Living.

If you are going to be spending much time in towns, with their narrow streets and challenging parking (!), though, you really don't want anything bigger then necessary (I listened to an American group in Seville just last Friday going down this route, as they had booked a small SUV, and rejected what they were offered as "far too small", and they ended up taking the largest vehicle on fleet - they will have a nightmare with it in the narrow streets of Andalucian towns, which the Hertz agent tried to explain, and which they rejected as nonsense!).

So, yes the Giulietta or some other C-Segment hatch will be the most fun to drive, and if you can cram your luggage in it, that would what I would take. If I were you, I would book one of those, and worst case, at the collection point, accept defeat and upgrade into something slightly bigger.