Hondo - I'll see if I can get all your ????
Good price for the pump!
150-400ft 1/2" tubing no problem for your pump.
NOTE: the 1/2 blk poly from Home Depot is true 5/8" ID not 1/2" as printed on the skin.

Extension cord will work to the pond but it is dangerous esp with snow and melting & rain water. By all means plug the damn thing into a ground fault plug. Be careful your wife may be a widow (let her know I'm single & looking!). And you might get some fried duck if there is a short.

I don't think the extension cord, if proper guage wire, will cause any extra electric usage.

For aeration of your small pond for ducks , it won't make a big difference if you use air just out the tubing or add a diffuser. AS Cecil said "Trust Me". I still would like you to try just tubing this year. This way you will also get a better idea of the benefit of a small bubble diffser. The winters may not be the same this yr and next yr but there will be enough temperature variation to get a good idea how big the open water area is at different temps, just estimate the opening size
and record the temps. If you use just tubing tie it to a brick or two so that when the air is on the tubing end comes up (rises) off the bottom about 14"-20". Don't release your air close to the mud/muck.

With duck fertility in the pond and an open water area present there will be more than enough phytoplankton activity in the water to keep your oxygen levels quite high all the time thru winter. Bubbling activity will have very little impact on the over all pond oxygen levels. Phytoplankton are doing most of your oxygenation!
Did I get all your ???

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management