The left thumbnail supplied by Raz in post #15 is what my scamp
seam looks like. I don't see any actual overlap. Just two pieces butted together with a layer of FG mat and resin applied to the inside, and I assume some resin between the top and bottom as they are brought together.
A lip on the inside in addition to helping align things if coming up from the bottom piece might act as a water barrier. However I'm not sure if one gets a stronger bond from having the surface on the inside all available for the FG layer that joins the two.
As pointed out the ridge line does impart some strength. But at least one with a vertical seam, the EggCamper
I think has double walls which I would think add strength.