I don't have any custom files for the game whatsoever (eta: except awesomemod and the framework), since last night was the first time I'd installed the game + EP. However, I will uninstall again, delete all leftover folders manually, and try reinstalling again. I'll edit this once that's done.
ETA: Yeaaaah. No luck. After uninstalling, I deleted the Electronic Arts folder out of Program Files. I reinstalled the base game, tried to patch straight from 1.0 to 1.8 (since patching from 1.6 to 1.8 had been failing), got the same Invalid File message. Haven't tried reinstalling WA yet, but before I uninstalled, that wouldn't patch to 2.3 either.
ETA 2:
Checked the fail log, the error is with "code_version.txt" (attached, in case that helps) so my best guess is that I'm using the wrong patch installer? The one I have is from 1.7.9 to 1.8, and my game is at 1.6 (which, btw, patches just fine). So just for chutes and goggles, I'm going to try to find a patch to 1.7 from 1.6 and see if I can patch from there. Will update when finished.
ETA 3: Oh sweet baby Jeebus, it worked! Obtained correct patches from here:
http://www.simprograms.com/the-sims-3-patches/patches-the-sims-3/ Made sure to compare the full filename, and updating from 1.6 > 1.7 > 1.8 actually WORKED. So, so very happy. Now I just need to hope WA patches all right once I install it...
ETA Last (hopefully!): WA patched just fine, installing framework and awesomemod now. Hopefully this will help anyone else with the same issue.