Thanks for the tips. I'm trying that Retract Continue right now (hoping it helps with #2 below).
I've run into two other issues tonight...
1.) If I enable Horizontal Expansion, the Z Seam Alignment setting gets messed up (I have Sharpest Corner selected). My only theory is that with the concave U shape, when it expands, some of the vertices get crossed, creating very sharp corners (this is not apparent when looking at the preview, though). So now the "sharpest corner" becomes part of the U shape somewhere as opposed to by the "feet" of the tunnel.
2.) You mention combing, and I've had this issue for over a year (as such I have been using Simplify3D, because this really messes up my prints)-----When it combs the print right before starting a new layer, it increases Z *before* the comb. I do not have z-hop enabled. This makes combing completely useless. All along that U shape I get strings galore.