Hi Nocash,
hope you enjoyed the coffee. Firstly as you know, the Com pin normally only goes low when the CPC is reading the Joystick or keyboard Y10 (at 1/50s as you stated). Theoretically yes, the z80 could set the 8255 pin (to the 74LS145) and the Com pin would remain low forever, but that's not what it does. Stick an oscilloscope on the Com pin when the Art package is running and you'll see it's not permanently low, so I CAN trust on this, because I didn't assume it, I measured it. So the pin going low IS indicating when the software is reading the port. Besides that, the software also reads the keyboard, so setting this pin low all of the time would mess up the keyboard scanning.
Correct, the buffer isn't a latch, but it doesn't need to be, the outputs of the 74LS240 are being read by IOA of the AY-8912 which is a latch, the reason for the 74LS240 is to ensure that highs and lows are only present when the Com port goes low.