Idasam,

As the model weight is about 5.5 pounds, and a 10G may be the max stress, you could test the joint by placing 40 or 50 pounds over the center, while the wing is supported on the tips only.

If still not sure, cover the bottom along the center with fiberglass and epoxy.

At least for me, 5 minutes (more like 3) is too short time for proper aligment.
If you have an incidence meter, just check that both halves have the same incidence angle.
It may be possible that the seller did not like the way the model flew due to a slightly twisted wing.

Best luck!