a mouse is NOT required to be used with a keyboard. the handedness of a keyboard is determined by itself.
How does hand dominance come into play on a device where both hands are expected to be trained to hit keys equally well? If your non-dominant hand is so wobbly that you can't hit keys on a numeric keypad, you might have bigger problems in being able to type at all!
How many "left-handed" keyboards are there anyway, and how many were around before the mid 90s? The only one I know of is the one pictured above, which if I understand correctly, is a Strong Man design acquired by Datacomp. (There's also the DSI Modular Mac, that arguably serves the same function.)
Don't forget that the function keys used to go on the left originally, which made more sense as you could hit them with your left pinky! It's not correct to assume any handedness with any keyboard based on the Model M, as people are simply copying a design from the 80s that's stuck, regardless of whether IBM had a clue. The top F key row was apparently only done to put the F keys below an on-screen contextual overlay, though it made room for more columns of keys (number pad AND navigation having dedicated keys).
One thing I struggle with when using my Poker II is having the alphanumeric area so central, as I guess I normally push the keyboard to the left to make room for the mouse and to not have so much of the keyboard so far out of reach; on the Poker I sometimes press keys one column over as a result. Personally I think that we should have the numeric keypad on the left, and navigation on the right. It's a compromise: the alphanumeric area is now central, it's "tenkeyless" as far as the mouse is concerned, and you still get numeric data entry speed.
Then again, despite believing that I couldn't live without the number pad, I mostly miss it on my Poker II simply by when I whack right shift when imagining that it is numpad enter ... My biggest confusion is mixing up Fn+A (left) with Ctrl+A (home in Linux) and trying to get home from Fn+A ... (that and misreading "PN" as "FN", which is really annoying).
The problem with the industry is that most companies prefer to just screw around with layouts in ways that don't really make any sense. The "left-handed" design is another weirdo idea that doesn't actually solve anything.
