Eh... I didn't think there was a problem with the time frame of the falchion. As far as I understand, they were common as muck during the period, and popular with commoners because, just like with axes, you only need a little good steel for the edge, making them cheaper than swords, and with that stiff, heavy blade, they could be used as tools just like machetes. That's apparently why there are relatively few left. Makes sense that they would be used a lot in war too by people unable to afford more dedicated weaponry. Compare with the popularity of machetes in modern african conflicts...
I think that in .960 at least, the native Falchion is not a weapon to be frowned at. It cuts lie a saber or high-end sword, and if it is a little slower and with shorter reach, that is to be expected by a short, very heavy blade, right? Only thing it really lacks is some thrusting damage. Is it realistic? You tell me, but I would be surprised if it's greatly underrated.