Can you import stuff from this into VTTs like Roll20 or Foundry?
No, this is an alternative to VTTs for in-person play.
Why can't the secondary screen be deactivated in the Main Menu once activated?
This is a limitation of the Unity 3D Engine we are using as the basis for Infinite Realms. You have to restart the software to use single screen mode in this case.
Infinite Realms is behaving strangely in windowed mode
This is a limitation of the Unity 3D Engine we are using as the basis for Infinite Realms. Multi screen is only possible in fullscreen mode, so windowed mode is unsupported.
Some maps seem to be unfinished / are missing thumbnails
There is a small [WiP] - work in progress - tag on these maps as they are not finished yet and only available as a preview.
Map titles overlap and look strange in the map selection menu
We plan to replace the map selection menu with a version that allows for longer map names
Grid and Sounds sometimes behave strangely when loading a map
There is an evasive bug in the map initialisation procedure. We have it on our list for a fix.
There is no sound effect attached to the snow weather profiles
Correct, there are only default sound effects for thunderstorm, hail and rain weather profiles. This is because snow hitting the ground usually makes no sound by itself - but you can pull up the wind slider for wind sounds and select some ambience sound in the Volume and Sound tab to complete your soundscape!
Can I use stuff from Dungeon Alchemist or other map making software in Infinite Realms?

As long as the map maker has some kind of image or video export. these files can be imported an played in Infinite Realms.

What makes this better for projection specifically, compared to other VTTs / map software? 

Infinite Realms is no VTT as it has no online functions. It is optimized for delivering battle maps (image maps, video maps or other images, e.g. handouts) for in-person sessions using  TV flat on the table or a ceiling mounted projector.

Does it have line of sight or just the fog of war eraser mode?

Line of sight and distance measurement tools, as well as a paint layer for markings and notes are our Kickstarter goals.