Welcome :)
1. Yes, with an Ultimate license your players do not need a license, they can play with the free/demo.
2. Yes, but we don't know what the upgrade price will be. Personally, I would get the ultimate subscription now and then when FGU is release get the FGU one-time Ultimate license. FGU is expected (not promised) in December of this year.
3. FGU is planned to run on MacOS Catalina. Their is currently a closed beta that is running, but it is on Windows only at the moment. But, FGU is planned to support Windows, Mac, and Linux.
4. Nope, but you might want to. Read;
https://www.fantasygrounds.com/forum...eed-to-play-5e
Let us know what other questions you might have.