Ah, right, yes. There's lots of PS2 keys with multibyte scan codes. Some with as many as 6 bytes! That shouldn't be the issue here though. The code synchronously reads in the PS2 codes in the main loop, and the ADB and pre-ADB stuff is all handled asynchronously through a timer interrupt and some context state that keeps track of things. I believe most of the issues are between the adapter and the computer, not the adapter and the ps2 keyboard.
For the forward slash, there's some ambiguity in my reading of the Guide to the Macintosh Family hardware on how the original Mac keypad behaved and the Plus keyboard behaved for those keys. I believe the original Mac keypad had arrow keys where the forward slash and other problem keys are, and you pressed shift to get the forward slash instead of the arrow key. Then the Plus keyboard inverted the case sense since it had separate arrow keys? This is the part where I get fuzzy and wish for a physical keyboard to compare against. So, it is my understanding that with the current behavior the physical keys on the PS2 keypad are being mapped to the same keys in the same physical location on the original Mac keypad. Whether that's 'correct', or what the correct behavior should be is a little more ambiguous I think.
For the missed key events, do they happen when you mash on the non-keypad side of the keyboard, or is it limited to keypad. I have different depths for the keypad vs. keyboard.
Thanks for the feedback!