Tube length should be fine.
The wing structure could be even better if the spars and vertical balsa filling would extend to main spar. This would form a triangle and reduce twisting of wing rib at the tube end area.
Weight increase is negligible.
Sure, more internal structure will make the wing even stronger. The question is where to stop adding more structure
My first design (Radiance) didn't even have a secondary spar, just the plywood rib doublers and i didn't see any sign of structural problems. Other F3A designs only have a plywood doubler at the end of the wingtube and no reinforcements in the wingtube area, so by comparison the Vortex wing should be slightly overbuilt. But the final weight was pretty good, so i don't regret adding the extra strength.