Just to close this out as we thankfully eventually figured it out -
Client 1: They already had a working 25-pin to 9-pin cable and we (wrongly) assumed it was the same as the standard IBM/PC 9 Pin I/F Cable we typically use. We ordered them a replacement and it worked. Still not clear how the old PC was able to use this other cable to communicate and we weren't, but that's OK.
Client 2: We typically use the built-in Serial1.readStringUntil(<'newline'>) method and it just works, but for this older CAS we instead had to use the recvWithEndMarker() method to receive the individual characters as outlined by @Robin2 here: Serial Input Basics - updated.
Appreciate the community's help.