I would try locating a point on the center of the distal stem, then align that point with a datum plane set 5mm + the radius of the stem above the green surface, Simpler math for NX and it won't go negative.
Check your sketch. I created a sketch with 3 circles (2 identical) and extruded the feature curves. I got 2 solids from one extrude. Maybe the sheet body comes from an almost complete full arc.
It doesn't happen on exiting the dialogue box, but it seems like the measurement function does lock up my computer for a minute or so maybe 10% of the time. Haven't really noticed any other hang ups.
NX1953.
I used the edges to create a ruled sheet body but isoparametric curves didn't work. Instead, I used Offset in Face to offset one edge of the ruled body half the face width, then created a tube that essentially matches your original. Part file also attached.
I downloaded a STEP file of a spring and used the ruled surface/isoparametric curve approach to recreate the centerline. I used the isoparametric curve to recreate the tube and it seems to work.
I tried to create a Tube Centerline using both faces of the tube but it failed. Tube Centerline does...