Since the mods and 'volunteer' mods go MIA, i'll easily answer this for you.
1) You do not need to buy another 100euros worth of ARK. You don't even need xbox live to play multiplayer from a PC if you buy the PC version from the store (gives you the xbox version), but you do need it to play the xbox version online.
2) Correct, you cannot host a dedicated and play from the same machine on either PC or XBOX. Unless you plan on playing when he's not, or have like 20+ people connecting (the reason for a dedicated server), then you should have him host a non-dedicated session on the PC so you get the benefits of no tethering etc. The thing is, it's not working right now, invites to friends, among just trying to join from the xbox or the PC xbox app for Windows 10, it's not working yet, but when they get it figured out, you're good to go that way)
3) This is unlike any other Play Anywhere title, and thus far not in a good way. It's supposedly "out" for 4 months retail, then Play Anywhere added later (Dec.14th was the real release for the Windows 10 version), but it doesn't work Cross-platform right now.