Yes, you can.

Yes, you can enable it with your current campaigns. But you can't disable it later.

I suspect that Steam achievements are not possible with this mod.

Sorry, I don't know. Suggesting to copy regular one to Elite. You will have two same branches.

<your steam library folder>\steamapps\workshop\content\261550\2886428498\settings.xml
Be aware that settings.xml is part of the mod, so it could be overwritten with updates. Sorry, I don't know how to fix it.

Yes, it's possible. There will be a line in town menu that allows to switch between recruitment pools. But I'm not sure what will happen with your clan mates.

Sorry, I never test such. You may try yourself and report if it cause problems. Can't give any promises that will be able to fix - I haven't sources.

Only Windows.