Originally Posted by
Often1
It is unlikely that NH will treat the transfer between two tickets at NRT as a transit. This leaves OP "stranded" at MEX and purchasing a new ticket back to the US.
I'm not so sure NH would veto this.
But what may make this easier if OP books MEX-NRT-ICN/ULN/IST (somewhere you could go with just a negative test) on one ticket and then NRT-EWR on the other. That is still a valid transit in NRT (unlike USA - Japan - USA) and so what if "plans change"?