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The problem is that a straight clubface hitting the ball on this out-to-in swing path doen't make it go straight, but results in a pull. So I guess the clubface has to be a little open, but not so much that it results in a slice? Maybe it's the D-plane that makes it go straight occationally ?(Which I don't know anything about).

In this instance, you club face needs to be square to the line of flight, not your plane line. I assume that you are speaking of iron shots, where you hit the ball before low point. If the face is square to the plane line traced on the ground, you will get a pull, probably a pull hook. In any case, you have to square the face up more than you have been in order to eliminate the fade/slice.

I am by no means an expert and am speaking from my experience. I play with an open plane line and hit the ball straight when I get my face squared to the target line (not the plane line), and hit fade/slices or pulls when I don't. Another fault of mine is to allow my head to move forward in the swing resulting in a plane line that is even more to the left than I intended and moves everything forward. That will also keep me from squaring the club face as needed, resulting in either weak fades or pulls.