How do I keep leather edges sharp when joining curved pieces?

The rounding at the seams usally comes from the leather itself being too thick at the joint, not from your bending technique. On 2oz it's less of a problem than on heavier stuff, but where two panels meet, you've got 4oz of material stacked up and the edge has nowhere to go except outward. Skive both joining edges down to almost nothing before you glue. Even taking half the thickness off makes the seam sit flat instead of bulging into a soft little roll. For the ninety degree joints, cut your pieces slightly oversized, glue them up with contact cement, and then trim the corner after assembly with a fresh blade. Trying to cut perfect angles beforehand never quite works because the leather shifts a hair when it bends. Trimming after the fact lets you clean the corner up exactly where it sits.

Once it's glued and trimmed, set the crease hard. A bone folder or the back of a steel ruler worked along the seam while the leather is still slightly damp will burnish a sharp line into it. If the piece has fully dried, dampen just the seam with a sponge and go over it again. You can also run a modeling spoon or a dull edge beveler lightly down the joint to define it further.

For the curved parts, don't fight the curve by hand. Form the panels over something solid, a wooden block shaped roughly to the profile, or even a section of pipe for simple curves. Let them dry completly on the form before you ever glue anything. Leather that's been heat-set or just fully dried into a curve holds its shape way better than leather you're bending live during assembly.

Last thing: case your pieces evenly before you start. Uneven moisture is what makes one corner crisp and the next one mushy, and it's frustrating because it looks random when it's really just the water content.

Gotta push back on the contact cement part. On a tiny detailed build like that it grabs instantly, so the second a panel touches down you're stuck with whatever angle it landed at. I do small stuff like this with regular white glue or wood glue instead, gives you a minute or two to nudge things square, and on 2oz it holds plenty once it's dry. If the joint needs real strength later you can wick a little super glue into the seam after it sets. Also not sold on trimming after assembly on thin stock, the blade tends to tear the skived edge instead of slicing it clean, at least in my hands.

Did a couple of those little house scenes back in the day and the corners only got truly sharp once I gave them somethig to wrap around. Tiny strip of hardwood or even a dowel glued inside the corner turns a mushy fold into a hard edge, leather has no choice but to take the line. The old saddlery trick works too, lace or saddle stitch the joint and pull it tight while damp, the thread acts like a vice and crimps the corner in as it dries. Slower, but you can't argue with the result.