I got it working by counting down from 255 for the entity ids in mars.conf.
Edit: I started my survival game last night, but I came across a structure right near my initial spawn point that had some densely-populated chests in it, which would have been balanced by the indestructible (at least as far as I could tell) monster spawners nearby if I wasn't playing in peaceful mode. It looked like some ruins, so I pulled the Ruins mod, but that didn't get rid of it, so I also pulled the New Dungeons mod which did eliminate it. (I re-created my world both times using the New Dawn world type and my username as the random seed)
Further Edit (presuming that post bumps are frowned upon): I ran into some trouble with Black Granite from GregTech - on my 4k TV it's very hard to see, even with the contrast set to 100 and the Gammabright mod installed and cranked up to 1500% (which is partly an issue with the quality of my TV). I figured out how to disable black granite in WorldGeneration.cfg, though I've come across a couple of other things I'm not happy about (which I don't see easy ways to deal with via config):
1. NEI won't show me the recipe for the standard IC2 solar panel, so I'm guessing it's disabled with GregTech installed, but that means I won't be able to build Compact Solars.
2. I read in the Q/A of the main GregTech post that GT machines can't take power via IC2 cables or from IC2 energy storage blocks. I'll take Greg's word for it that supporting them would cause lag, but I have to wonder whether GT really still qualifies as an IC2 addon when that conflict exists.
At the moment, I'm also a little overwhelmed by the number of materials and machines added by GregTech: e.g. 77 different plate types I can use to make a file? I'm also puzzled that I apparently could make a file with lead plates (according to NEI at least), but not with tin plates. Given enough time, I could probably get used to that, though.
I've also come across a detail of the New Dawn mod that I'm not entirely happy about: the beaches tend to have much less accessible sand (i.e. that doesn't require getting into the water to dig out) compared to the default worldgen's beach biome.