1. I would expect 100W/channel in stereo. After all, it is a NAD and NAD rates its equipment very conservatively.

2. Don't do it. There's no telling what the internal circuitry of the NAD is. The other amp could, for example, end up shorting the signal through its power supply capacitors.

3. There is no question 3.

Opinion: free power? Go for it!

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated