See this post which has essential info if you're trying to change core stuff:
https://forums.elementalgame.com/434780/page/1/#3378469
1. Is there a way that I can overwrite the core files, especially the racial blood traits, using the mod folder?
But basically... unfortunately an Ability can not be overwritten from the mod folder... you can *add* stuff just not remove it. So if you want to add something extra to a blood trait you would do it like this from the mod folder:
<AbilityBonus InternalName="Blood_IroneerAbility">
<AbilityBonusOption InternalName="Blood_Ironeer">
<GameModifier>
*** Do something here ***
</GameModifier>
</AbilityBonusOption>
</AbilityBonus>
2. Do I need to keep the same folder structure in the mods folder as in the core game folder?
Nope you can do whatever you want with folder, files etc under the Mod folder.
3. Can I name my modded XML files the way I want? How does the game find the right files then?
Call them whatever you want... it reads all files under the Mods folder... it looks for XML tags like <AbilityBonus> etc.
If you haven't already take a look at a few small mods to see how others have done it. Parrotmath has a lot of fairly small mods that would be good examples.
Also, I wanted to overwrite the splash loading screen. I found a file named accordingly but the old one kept displaying no matter what I did. I suspect the screen is somehow in the executable or did I miss something?
No idea about that one but you got me curious i'll take a look later.
Edit-> Yeah doesn't seem to work from the Mod folder. If you change it in the Core folder it works fine.
