I couldn’t find anything about it:
Will I have to have an account on each platform to be able to play on all of them or will I have to buy a game for each to play on both PC and XBX/PS?
Another question is whether in the case of consoles, will I be able to play with a friend using only 1 account?
you need to buy the game on each platform.
but its all cross play, so you should be able to play with your friends while you are on PC and they are on xbox. but to play on xbox with your friends, you need an xbox copy.
there is couch play, so if your friend has D4 on xbox and you play as the 2nd player (on his xbox), you dont need a copy of D4 on xbox, but you need to log into your xbox account.
now my question is… do they need to have a bnet account and linked to their xbox/ps? or is their xbox/ps handle all thats needed?
If you want to play on each platform, you need the game for that platform. Then you would need to link your Xbox or Playstation account to your Battlenet account in Account Management here on the website.
Once you do that, progress you make on any of those platforms is all shared.
To PLAY with someone on another platform you don’t have to do anything except have the game on one platform. I can have it on PC and play with someone who is on Xbox. I don’t have to buy it twice to do that. We just meet up in-game and play.