Hey jphil,
I might be a little late to help at this point, but I would agree – it’s probably a combination of both. My earliest dags (granted, they were silvered glass and not regular plates) were poorly polished, and generally very hazy. This resulted in negative-only results, even when they were developed properly.
I’m sure underdevelopment didn’t help all that much here, though. Try not to worry about heat — mitigate it when you can, but honestly I’ve left mine sit out on some pretty hot summer days and haven’t really seen noticeable amounts of fog. Look at it this way – underdeveloped plates are almost entirely worthless as images, so you may as well commit to the full development!
You pulling it early reminded me of my first few – I did that too! I was just too excited and couldn’t wait any longer. It always looks so much more dense when under rubylith than what it really is. If you’re really worried about development times, you can take it inside and develop with a floodlight & fan for a more controlled setup while you learn the process a bit. It might be worth tracking down some amberlith if you can, it cuts the development time about in half vs rubylith.