I tried "creating mineral fields at a location" for player neutral or neutral player, but they don't show up? They do for other players though.

Also, is it possible to create more than 1 mineral field at a location? I tried "create 3 mineral fields at location" for player1 or neutral, but only 1 shows up.

To your second question
I'm pretty sure that mineral fields handels like buildings, they can't be mass created at 1 location.
You can create a chunk for player 1 and give it to neutrals (?)

I thought if resources were created for a player, it would automatically be given to the neutral player. Needs a test.

Not automatically. But it's np to add a give action after the creation action.

Create them for a player and they will show up for p12 in the map (without give action or anything else).

For multiple ressource fields, you need to move the location around or use multiple locations.
It's the same with buildings. You can create them only in the middle of a location.

You cannot create units for player 12.