OK, so I finally got around to trying it out for myself. But as usual, it didn't play nicely with debug mode. These are the errors (some rather minor) that I found:
Line 70 of monarchs_brl.txt needs a #
Line 115 of monarchs_brn.txt needs a #
Lines 48 and 64 of major_ErnestineSaxony.txt contain double quotes (")
Lines 2918-2921, 2944-2947, 2956-2959, and 2983-2986 of major_Germany.txt have "owned = { province = X which = LAT }" when it should be "data = LAT"
Lines 3102-3106 of major_Germany.txt have "owned = { province = X which = GER }" when it should be "data = GER"
Line 1600 of major_Poland.txt contains double quotes
Line 19 of major_Pomerania.txt is missing an equals sign
Line 41 contains double quotes
Lines 2020-2023 and 2040-2043 of major_Prussia.txt have "owned = { province = X which = PRU }" when it should be "data = PRU"
Line 2112 contains double quotes
Line 12 of major_SaxeLauenburg.txt is missing an equals sign
Line 193 of major_Transylvania.txt contains double quotes
Line 2721 of provinces.txt has an extraneous "march"
Line 12909 contains double quotes and is too long
Line 552 of special_HYW_Feuds.txt contains double quotes
Line 12163 of randomevents.txt has an illegal character (¨)
Line 5 of GFX\Map\Units\T-ARMY C-CYP A-FIRE D-SW L-1.spr is invalid
Line 802 of revolt.txt is invalid (PFA needs a capital)
MOC (Modoc), ASR (Australia), LOU (Louisiana), and BLW (not going to try) are defined in revolt.txt but not countries.txt (and Australia needs a capital)
Line 2989 of revolt.txt is invalid (MKS has two capitals)
A number of provinces in provinces.txt have invalid areas: America, GreatLakes, Acadie, Hudson Bay, LaPlata, Malaya, Liberia, Cape Verde, St Helena, -, or have region = "Lake"
PIR is defined in the 1337 scenario (possibly others) but is also included from Inc (although this isn't necessarily a problem)
On lines 68689, 73377, 74327, and 78179 of the 1337 scenario, there are land units with an invalid type. The type code for a unit has to be at least 4712 (don't ask me why).
WATKABAOI is going to be a nice stress test for the performance improvements I'm trying out. If there's a place where something is unnecessarily done for each pair of countries, having 600 countries will make it stand out (600*600 is quite a lot).