> 1. Is the unfolded screen a continuous flat surface? (I.e. is the vertical bulge in the center gone?)

Nope, no manufacturer has solved this. There is a "crease" in the middle the pixel fold here which is in the middle between the Z fold4 and some of the newer Chinese phone companies that do a "tear drop" folder to limit the creasing effect. So it's noticeable but eventually you'll get over it.

> 2. Are there gaps around the hinge or screen edge? Is the phone weatherproof?

Two questions, yes there is a gap top and bottom in the middle when unfolded. A lot of the newer phones are weatherproof yes but I wouldn't take any phone out in a tornado or leave it lying in the sand at a beach.

> 3. Are there gaps when the phone is in the folded state?

Gaps? Where ? When folded you can think of it as two phone sitting directly on top of each other and attached at one side.