Great points, sjpt. Thanks for making them. :-)
I made a PNG of exactly 600 x 800 pixels at 167 ppi (the size and res of my Kobo's screen). It kinda works, but ADE 2 still wants a border of roughly 30 pixels on either side of the displayed page, and if I try to shrink the window beyond that it starts compressing the image horizontally.
Oh well, it'll do as a rough yardstick. I always knew I'd have to load the file into the Kobo for final checks, but this will let me get most of the work done much more quickly, and then I can do the finetuning by checking it on the actual device.
And from what I've read, ADE doesn't really behave the same as any hardware reader — I suppose it's unreasonable to expect it to, but ePub is a strangely amorphous "standard" when it behaves differently in almost every display method. And I've (perhaps unrealistically) fallen into the habit of expecting Adobe to set the standard, or at least reflect the standard, on design and display matters.
Ah well, should get on with it. Thanks again for your help! :-)