I’m sure you could go ahead and stitch then tape and epoxy the first seam without the other two panels in place without a problem. The indexing in the mold and wiring it down will hold its position and shape. However, I followed the steps in the manual and had no problems accessing the center seam by tipping the mold from bow to stern a little. The hardest part with the center seam was actually towards the front where its almost vertical. Tipping the mold by placing the intercostals on some paint cans made that part a lot easier (just take caution not to rack the mold frame at this step, keep it square and aligned).
I will also add a couple of tips from experience. The fiberglass tape has one edge where the weave loops back on its self called the selvage. I used a nice sharp pair of scissors to carefully trim this off just prior to laying down the strip over the seam. The selvage ends up creating a thicker edge when epoxied which will require more sanding later. Keep in mind that if you choose to go this route you will have to be a little careful of pulling loose threads.
The second tip is for when you get to the curved seams and even applies to the large fiberglass step later. Fiberglass cloth will conform to a curved surface easier if you lightly “pat” the bubbles down with your hand as apposed to forcing it with a tool or stretching it. Start from the center and work your way out by lightly patting the fabric and let the fibers move slightly to conform to the curvature. Use pushpins if you need to hold it in place but NOT tape. Tape will leave a residue and pull up loose threads.
Good Luck, and keep us posted on your progress.