Guess the problem is somewhere in your system setting or antivirus since it seems not to allow downloading apps outside the play store.
I am able to load other apps outside the play store.
Receiving "package appears to be invalid" error when I install. Can you share the apk directly or differently?
What version are you trying to install? The free one (Episode 1-4) or the full game (Episode 1-6)?
1-6
1-4 seemed to download fine
Please completely remove 1-4 before installing 1-6. Contact me on discord if you still have problems: eXtasy Games#5399