Oh wow. Now I'm confused. A screwed up auto height sensor should not affect manual controls or settings - but clearly is. Honestly, no clue! At this point, I'd be really interested in how your combine works on another header (anything) - does it still drop like a rock, or does it work correctly? If it works fine on a different header, then its got to be something in the header or settings. If it still drops like a rock---then its a combine problem and the failure just happened to be just right to make it go into "auto/slow mode".
I'm wondering if maybe your lift pressure sensor is not working right....hows the left bar graph? On a cornhead, etc, It should read 0 with the head fully resting on the ground, 50 with the head frame just lifting off the ground, and 100 with the sensors all clear of the ground. This is hard to see on a MacDon---as the spring float messes up the simple explanation -- but you should still be able to get the idea. Make sure the 0-50 range seems to be working right...that is your lift pressure sensor.