Obviously, this is too late now for OP.
About 12-18 months ago, at EVA counter, another passenger was having the same issue (CSR does not fully understand the TWOV rules). In that case, the itin was SFO-TPE-PVG-USA. The compromise I helped to broker was to get him to be checked in for SFO-TPE only and the passenger would sort it out in TPE.
If OP offered this solution to DL, would that have been accepted?
Also, it is amusing that they consulted a CBP guy. Why would CBP know any visa rules of any other country?