My ideal keyboard would have buckling spring switches in the body of a Poker 2 for this exact reason. I can use a separate numpad on the far side of my mouse for extended numeric entry, but the far more frequent use of keeping my left over the keys while my right uses the rodent is made much less RSI-prone.

When you had your shoulder pain, was it primarily at the very base of the shoulder blade? That's where it was for me before I moved to a vertical mouse.