I just haven't figured out how they lay up balsa in the mold without using a ton of resin; or, how they vacuum bag a fuselage without collapsing it.

Imagine a female mold for each half of the fuselage. The resin/cloth is laid in first. The balsa skin is then laid in. The vacuum bag is applied and forces the composite material onto the mold surfaces. When cured the halves arre removed and joined along the seam, maybe in the mold halves clamped together.

Ed S