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From what I have gathered, no easy task to do this, many GIS programs seem to struggle with pinpoint accuracy as well. The method for QGIS, is involved, so I didn't get time to run through it with the plugin, but I did dig into reddit and some GIS sites. Basically they do similar method, they just add a lot of points down the "polyline/line" and run the appropriate "centerline tool". Where someone posted results, I saw some in some of them the same "off center through some areas" as in the methods posted here. This was apparently plenty good enough for most GIS users, though I only did a relatively quick dig. In a similar method for AutoCAD, maybe adding more points to the polylines would help. I might have time Monday at home to check QGIS. Though I did import the OPs drawings into a session, the first one comes in very strange, the second seemed to be correct. The OP still hasn't answered why the second .dwg posted comes up as not an AutoCAD drawing. For AutoCAD, it helps if BOTH polylines go the same direction.

My $0.05 does not check for off or frozen layers. (defun c:wow ( / ent co-ord ss) (if (tblsearch "layer" "Site") (princ) (command "-layer" "Make" "Site" "c" 1 "" "") ) (if (tblsearch "layer" "Exist") (princ) (command "-layer" "Make" "Exist" "c" 2 "" "") ) (command "chprop" "all" "" "la" "Site" "") (setq ent (car (entsel "\nPick the pline "))) (setq co-ord (mapcar 'cdr (vl-remove-if-not '(lambda (x) (= (car x) 10)) (entget ent)))) (setq ss (ssget "WP" Co-ord)) (command "chprop" ss "" "LA" "Exist" "") (princ) ) (c:wow)