If you create a mineral field for player 1, it seems that it is owned by player 12. So is it possible to create mineral fields for player 1, owned by player 1 and create it directly (without using: give unit)?

You can preplace units with a map editor that are owned by specific players. As for creating in game resources owned by specific players, you'll have to test that out as I've never bothered to try.

Well, SCMD gives you the following warning message when you place a mineral field for a player other than P12:

"Resources placed for player other than neutral. May not appear in StarCraft."

Not sure what it means. I don't think it would do anything more than crash you or glitch something.

Preplacing for player 1 works ok, I don't get the message or any crash either. But I can't get ingame creating minerals for player1 to work.

If you can't create or give it, then it can't be done. You'll have to make do with preplace or player 12.

I'm just wondering what use you would need for a p1 mineral field that couldn't be accomplished by using a p12 mineral field?

My point exactly. You run into problems that aren't worth the benefits, none of which I can see (if they even exist).