It is very common in serial communication ports to combine the frame ground (FG) and the signal ground (SG). That doesn't surprise me at all that this was done in this instance. I don't like this practice, but I digress.

It is also very possible to twist and tin these combined ground conductors together and form them into something that will work in a crimped modular plug. Comdial did this with the equivalent of speaker wire from their 'wall wart' power supplies for 'Solo II' phones. It can be done.

As to why your reconstruction of this cable working; I just can't say a thing. It sounds as if you have covered all of the bases. I'd focus upon improper crimps where conductors were either missed or they jumped over upon insertion.

Ed Vaughn, MBSWWYPBX