You don’t need the cable to use the card. I don’t think there is any real reason the cables are expensive, beyond the fact that they seem rarer than the cards (people toss unusual cables all the time; less so installed expansion cards).
I don’t think you can use disk images, but you can use the computer’s mouse and floppy drive, and you can put ProDOS partitions on the computer’s hard drive. As far as I know, then only case in which the Y-Cable (or a replacement) is strictly necessary is for the joystick.