Deleting non-manifold Edges?

Support Modeling
#1

This mesh has a number of problems, including non-manifold Edges. I can Select them using SELECT BY TRAIT, but when I use DELETE or DISSOLVE EDGES, it messes up the mesh.

What is the proper method to kill these bad Edges? Tnx.
NONmanifold.blend (600.7 KB)

#2

You can fix some problems by “merge by distance” and check on “merge” for the mirror modifier.
But as far as i see the mesh does not suit well for subdivision modifier yet. But i am still a retopo noob so maybe anotherone can tell more here.