So, for a graphic which is 20px tall (per frame), but has DEFAULT_HEIGHT_16 specified in the .nxmo, I understand that this will cause the graphic to appear 16px tall in-game (in order to preserve water objects in existing levels), but - will it appear as 20px tall or 16px tall in the editor (i.e. specifically at the point when drawn)?
You're overestimating how much the DEFAULT_HEIGHT parameter does.
It does exactly two things, and
nothing more (and similar for DEFAULT_WIDTH):
1. When loading a level file that contains (for example) default:water, but does not specify a height for it, DEFAULT_HEIGHT is used for the height. (If DEFAULT_HEIGHT is set to whatever the old size was - in this case 16 - this will mean levels do not break - because they expect the water to be 16 tall, and it is.)
This applies to both the game and editor.2. When placing a new copy of the object in the editor, it will initially be DEFAULT_HEIGHT pixels tall, again, 16px in this case. Same size it always has been upon initial placement.
There's no reason why the same object, with the same width / height (whether it is manually applied, or just the default), would look different in the game vs in the editor, beyond the normal game / editor differences (eg. editor doesn't support alpha blending). In particular, they are not going to draw it at a different height. Once the object is placed, and its size is loaded (using the default, or the frame size, as fallbacks), the DEFAULT_HEIGHT parameter no longer does anything, and the renderer never knows it exists nor what its value is.
I should note to anyone reading this - if you're getting confused by DEFAULT_WIDTH and DEFAULT_HEIGHT, but don't actually need to be using it (and aren't simply curious about how it works), the answer is pretty much "don't worry about it". You can continue doing what you've always done, and there's not much reason to use them on
new objects, they're more a backwards-compatibility thing if you have to
change the base size of an existing resizable object.
Regarding the graphics - sounds good based on your explanation, will actually take a look later.