Can I play with my friends from UK/US from Japan???????

Yes you can still play with your friends from the US/UK/EU. You can use party chat to party up with those friends and play multiplayer games like Call of Duty and Battlefield.

I don't think we are going to see it here other than the PC. Japan gets it in Sept but it's no where to be found on any US release calendar.

It's not region locked and I have a friend who picked it up from Game in the UK. He said the shipping wasn't expensive.

It depends, Xbox is region locked, each region has a code, I own a Xbox from the US, I visit Japan often and my Xbox does NOT play MOST games from Japan. Here is a list of regions hope this helps!
North America and South America NTSC-U. Japan and Asia (Except China) NTSC-J. Europe, New zealand and Australia PAL region. China NTSC-C. Most games will not work between Spain and the UK, but Microsoft is tricky because they let
each publisher choose if the game is locked or not, like assassin's creed could be bought and played anywhere. I hope this helps some.

1) I'm not sure about that myself.

2) If you're using an account with a different region than Japan, then you can run into issues if you try to download region locked content. If you're using a Japanese account, then you shouldn't have any problems.

3) Points cards are region locked. If you're still using a UK account, then you would need to purchase UK points. If you're using a Japanese account, you'd need to purchase points from Japan.

The digital code from the US will work on a UK Xbox One. I received a Fifa code from a friend in the UK and it works perfectly fine on my US console