That is going to be a nice area for the pond, Lantern! Still raining however, hm?

A pond can be less work, yes, if you are planning it right. I also think that a bottom drain is a ton of work, and you need a lot of plumbing with it. We do have bottom drains, but our ponds are above ground, and there isn't much plumbing involved. Just the drain, a pipe that leads the water outside the pond, and gravity does the rest. We made the pond bottom with a slight slope towards the drain, and you can do the same, even without a drain being actually there. That way, the waste will collect in that little valley on the bottom, and all you do to clean it up is a pump, that has a longer hose attached. Turn on the pump when it sits right in that lowest part of the pond, and the hose will carry the waste out of the pond.
I like to think that this is the most important part when designing a pond - the slope, or a lowest point in the bottom. That is where the waste and most of the leaves will go, and you have eliminated a ton of time that you otherwise would be using vacuuming the whole pond bottom.

You still might have to do it occassionally, but not very often.
Rocks, yes, same reason for not using so many - things can sneak under and among them, and just sit there and rot. Not a nice thought...
Round ponds are not harder to care for, no. You get less water quantity, but other than that - whatever you like better.
When you do the waterfall, I'd suggest building the mount out of soil, making a 1 foot wide canal, so to speak (or however wide you want it), then line the whole thing with pond liner, and
then use all the rocks for shaping the mountain, and also to secure the pond liner in the canal. You can locate rocks so that the water flow in the canal isn't straight - kind of like a natural stream, where rocks guide the water into little left and right swirls. Of course, you can do the wall with rocks as well, but if you do part of it with soil, you don't hate rocks that much at the end of the project.

One thing that keeps going through my mind while looking at your last picture - the pond area itself isn't in the lowest area of the property, is it? SImply because when it rains and all the water comes running to the lowest point, you don't want the pond to be that point, and possibly overflow with runoff. From the looks of it, your area likes the rain...