2: Loading custom graphics for any map is a 'maybe'. The reason is that there are tiles, characters, and in the future units. I would have to have them all packed into one png to be easily loadable into any game. Can be done, just not a top priority.
Well, I hope you get around to it eventually. I can remember this from Boulderdash on my C64 and it was great.
3: Related to #2, you can save any of the built in maps and them modify them in the editor. So you could play Cubic with a different background and tiles, more or less enemies. Note that when you do this, it becomes a 'different' map and scores won't be posted against the original map. To do this, just press "p" while on the black title screen of a map. This will pause the game before it even starts. Click to start the map, then save the game into a slot. Grab the slot file and open it in the editor and you are in business.
I did not know this. Thanks for the info!