If you add that switch, sure, you can remove those watts completely, but if you were to move to an external enclosure, the whole enclosure + DVD drive would have to use 4 watts or less (to keep the system as low powered as possible).
In short, less than possible, considering what a power brick would use, and what an external enclosure would use, and what the DVD drive itself would need (guessing 4W by existing information).
Don't quite follow this. Depends on how much time the drive is in use as well as how much power/other resources were involved in the external enclosure.
By lagagnon's calculations, having the drive not be idle saves approx 35.1 KW-H/year. [BTW - it is watt-hours (watts times hours), not watts/hour (which would be watts per hour, or the change in rate of energy use)]
Assume that the external enclosure only uses power when the drive is in use (it has a real switch so it is turned all the way off, or just unplug it.) So if the drive were to use 35 watts when on (not a reasonable number - just makes the figuring easy), you could run it for 1,000 hours in a year and still use the same amount of power.
Using a secondhand enclosure (or a bare USB to SATA adapter) would help reduce the embodied energy and expense, and bring the payback time down.