Well, I gave the crushing idea a shot.  Since it seemed nearly impossible to remove the drive gear along with the cable (unscrewing the cable was hard enough trying to squeeze the hands thorough the tiny openings from underneath the car), I decided just to unscrew the cable and see if I could crush it without completely removing the cable from the speedometer as well.  Working underneath the car with my loose vice, it was clear I couldn't get much force on it to crush it much at all.  In fact, I couldn't even really tell if I had crushed and made it any wider at all.  So I decided to give up  and just put it back in and take my other cable to the speedo shop to be modified.  Surprisingly, that little bit of crushing was just enough to make it work.  Seems its a very fine line of it catching the sides vs not.  Anyway, I'm a little nervous that over time those edges might wear out.  So I might just modify the other cable, but maybe I'll wait to see how long I can get away with the current fix. 

It's a lousy design as you noted.  The long axis of the oval is supposed to be large enough to engage the diagonal of the square drive hole in the gearbox.  It was probably changed as a cost savings.

I see two options.

1) Take the speedo core to the bench along with the drive gear from the transmission.  Crush the speedo cable oval in a bench vise and check its fit in the drive gear.  When you are confident the cable fits without slipping, reinstall.

2) Take the cable and drive gear to a speedometer repair shop.  Have them make you a new core with the proper square profile on both ends.

As part of the effort to put back the original SPI dash & gauges in my car, I had to get a different speedo cable that was long enough and had the special fitting to engage the Nippon Seiki speedo.  I was of course expecting the other end of the cable that connects to the transmission to be the same.  As you can see in the picture, the cable that I took out is the dirty one on the left which has a square solid end which I presume engages a female square piece inside the transmission.  The new cable I got (on the right) has what looks to be just a round braided cable that is crushed into an oval shape.  Long story short, when I installed it on the car, it clearly does not rotate and so the speedo does not work.  I made sure by disengaging at the speedo end and observing the cable as I was driving.  I thought maybe this was a flawed cable so I ordered the same part from our host, and it basically looks exactly like what I had previously ordered.  Is this just a lousy design or am I missing something.