I had told magiclauncher to fetch the 1.4.2 jar, and the log I posted above seemed to match that, but given that I was able to get everything to work by switching to MultiMC, I'll grant that maybe something in the magiclauncher setup didn't properly switch to 1.4.2. Anyway, for anyone else who hits this problem the easiest solution is starting fresh with MultiMC... which lets you manage coremods and can download forge direct, so is better imho anyway.
Thanks for taking a look!