Hello! : )
About the first issue:
Maybe it’s a download issue. GitHub isn’t so friendly with unreal projects yet. : S
This button doesn’t work in this context, this will download all assets as “corrupted” files with only 1kb size and the project will not work:
The same will occur if you’re cloning the repository without Git LFS installed.
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If you’re cloning the repository, ensure that you have Git LFS installed and call this command inside the project’s folder after the clone:
git submodule update --init --recursive -
If you’re downloading the .zip, use this link: Releases · lucoiso/UEProject_Elementus (github.com)
Second Issue
The AbilityInputBinding.h was moved/changed/renamed to MFEA_AbilityInputBinding.h
Third Issue
That’s strange, there’s only one header with this name 
And here’s the guy: GameFeatureAction_WorldActionBase.h
Can you try again downloading a .zip file from this link: Modular Features Extra Actions - Releases?
If the problem still persists, let me know!
Edit 1:
The problem could be related to Git LFS as well, I noticed it was out of bandwidth which affects the repository usage because it disables LFS.
I bought 1 more data pack on GitHub to solve this.