"Any problems with having this 2x4 treated would in between the origional porch slab and foundation? (Homeowner installed a deck on top of slab)."


Not that I can think of

"Also, does anyone recommend L-flashing going up behind the siding and then on top of the first row planks to prevent water from deteriorating the bottom course of hardie plank?"

If that is Hardi plank, than it will not deteriorate because of water.