The Faction Selection crash occurs when you click on several factions on the Faction Selection menu and M2TW CTDs. This happens if a mod has more than 26 factions.
Below is my examination of this problem. This is for modders who want to know what I have tried.
The m2tw.Int contains the fix and allows you to have up to 31 factions in the Grand Campaign, Custom Battle, and Hotseat. The icons have been made smaller in the custom battle selection screen to accommodate all 31 icons.
This m2tw.Int replaces the Faction Selection Maps with the faction's special unit image. This is demonstrated in the screenshot below. The reason I have used this image is that M2TW can cope with 31 different Faction's Special Unit images but not 31 different campaign maps. As there was not other suitable image I had to use this one.
1) Download the attached m2tw.rar, unpack it, and put the m2tw.Int in your mod's 'data\menu' folder.
2) Your Faction's Special Unit images and Faction Selection Maps will now have switched place on the Faction Selection Screens.
3) Replace all your Faction's Special Unit images in your mod's 'data\menu\symbols\fe_faction_units' with the Faction Selection Maps in your mods 'data\world\maps\campaign\imperial_campaign'.
4) Delete all the maps from the 'data\world\maps\campaign\imperial_campaign' folder. If the files remain in here the mod will continue to crash on the Faction Selection Screen. MAP FILES IN THIS FOLDER CAUSE THE CRASH.
5) Your factions no longer have any Special Unit images but you can edit the 'data/world/maps/campaign/imperial_campaign/map_FE.tga' file which will add an image into the space where the Special Unit images used to be.
This fix doesn't fix the crash you get when you select the Grand Campaign, then go back to the menu and choose Custom Battle. I don't know how to fix it.