I looked at SM and rear seat and many panels and covers (inside and underneath) need removed to get to HVB cable where it goes through floor pan. Also, if they had to physically remove the cable from the vehicle to repair, it would add to the labor hours. I would argue with dealer that $160 an hour is extremely high. You don't need your senior tech removing nuts, bolts and a cable. I have not heard of anyone who had the same issue.
You are probably correct in the root cause is likely a sloppy repair 2 years prior. It's hard to tell from the SM image but there doesn't appears to be anything (loom or otherwise) holding cables in place as it bends and goes through opening underneath car. Perhaps the cable was pulled to tight and thus rubbed. Look at pic below as there is no loom shown going through opening.
I do know that some posters have had wiring issues with the HVB and Ford does not cover wiring past the 3/36, B2B warranty. Also, IIRC Ford does warrant replacement parts plus installation which were paid for by the owner for 2 years. So, since the previous repair was covered under the original warranty, there would be no 2 year extension to the original warranty.