Andrew,
I went through all of the EGR, Resonance Flap issues, and not once did they cause the LHM. I was always able to drive at highway speed, although it would surge like crazy. Sounds like you might have a bad battery or low voltage issue causing the LHM, and also have an EGR problem. Where are you located, maybe someone nearby can clear the codes if they have an OBD scanner. Also, to disconnect the battery there is a small square plastic cover in the pass. footwell, open that cover and you can disconnect the negative ground without removing the seat.
YOu might need to have the intake manifold off again and test the 2 resonance flaps. See image below
