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1) increasing the number of classes to 24 characters (that's what I use it for)
1. This is not done in the exe file. It is Lua code and can be found in MMH55-Index\scripts\H55-Core.lua
Hero definitions start between lines 1190 and 1217. I suspect there are a few preparations for it, but I don't know where they happen. Look around.
The new EXE supports it; without it, the game just crashes when entering the new classes. The original EXE does not work with 24 classes; it was precisely the reason for which I used a modified EXE file from the RPE project.