As long as Apple doesn't invest in it's own platform to make it attractive for game developers to work on, it won't happen.The unreal engine supports Mac OS, Linux and some consoles for that matter. Steam is the issue here, not the Unreal Engine.
I strongly doubt it.
The market for Mac games is too small, and it would take too much effort for a small indy company like TW to produce a Mac version, effort that would not be worth it.
It would take bugger all effort for TWI to support Mac and Linux, as the engine they're using supports it natively right off the bat as I have already expressly stated. But that's irrelevant because as I said... Valve only supports windows, and RO is a steam based game.
The ball is not in TWI's court.