What if they are slightly diagonal to the line to the hole?
Do we use the forward one for the reference for how far back we can go or the back one?
Unless, this is because some player accidentally moved it?
You make a rectangle. It doesn't matter where the hole goes - just that one side of the rectangle is defined, and rectangles are made of right angles.
Trouble is you can't know if it was moved accidentally or if that's how it was set up. The rules see the tee markers as fixed and immovable when playing from the tee, and as movable obstructions for any other stroke. I have seen players pull them out and toss them aside without bothering to mark the location, and I've seen them set up poorly by mowers who really don't know how to do properly. When the player steps up on the teeing ground, they are inviolate. If they are pointed slightly off, then it's up to the player to aim himself in the right direction.
The teeing ground is not a parallelogram, it's a rectangle, and the direction it is pointed is irrelevant.
You use both of them. I'm not sure how to explain it any better. they could be aimed 45 degrees away from the fairway but they still form a rectangle 2 clublengths deep from a line drawn straight across the front edges of the markers. Even if the entire rectangle is aimed 45 degrees off, you still have to hit from within that rectangle.
See this rough sketch. Direction of play is shown by the arrow. Both red boxes are legitimate teeing ground setups, but the back one is set up poorly. As a player you just deal with it, but you still have to play from within the red box.