Mirrored simple surface shows naked edge on one only

dear steve

some issues already discussed here:

the “_what” command on the initial wing shows an edge tolerance up to 0.002
your document tolerance is 0.001
→ _trim again
→ _join again
depends a bit on what you re after - but please notice:
if you use a real cone or an revolve the Edge (A) might not be a line.
if you do a sweep2 the surface will not be 100% rotational./ or not a perfect cone.

it s realy hard to guess where the inprecision comes from - but above workflow will fix your naked edges:
(i used your _sweep2 approach)

Naked edge yet A and B are mirrored_02.3dm (3.9 MB)

Naked edge yet A and B are mirrored_02_rh5.3dm (375.7 KB)

kind regards -tom