i'd go for the yellow line.
here's what my undertray looks like (ive since installed wheel deflectors and experimented with closeing off the gap at the sides)
if your car had a perfeclty smooth underside you'd actually want an undertray that angles up in the front and center, but wand to keep some sort of deflector in front of the wheels
the loremo is a good but extreme example of this
but as you can see in the pic of my car even there this trick is applied very subtle... and lots of cars have this today
so you might want to considder reduceing the height of the dam in the center.
on the other hand a dam with a higher horizontal surface behind it will couse a little downforce... this has no real benefit for FE, and it won't suck your car down but it can make that difference between an unsteady feel and a solid feel at speeds
so my final advise would be to install a tray that slopes down from the original bumperline to the lowest point of the car, like you seem to have illustrated in your drawing, and that perhaps retain a portion of the dam in front of the wheels and perhaps suspension components