It's interesting you want to
improve the functionality of the Eject key, where most users seem to want to
repurpose (read: do away with) it. While it makes perfect sense to me, I'm not sure how to remap the Eject key to meet your goal, assuming it's possible. Fortunately, there is a working software alternative: the
menu bar Eject button/icon. Clicking this will list the available drives in a pulldown menu, allowing you to select the one you want. So yes, you still got to make a choice instead of just hitting a key, but it does do the job.
If it doesn't matter which drive is mounted in the top slot, the second item on your wish list sounds like a slot-hardware issue; perhaps the
soft rubber anti-vibration drive grommets are worn or missing from the top slot. If true, you might want to replace the grommets in all drives, they aren't that expensive.