Over reacting on the conduit. Nailplate its path or remove the plywood so no lags go there. Little over kill on the plywood anyways. Get your low voltage ring in the wall and you will have a reference picture for everything. You know all that anyways. Electrical problems I would not worry about either. Besides, the way it sounds you will be paid and long gone before he gets there to use it!

I would he more concerned with power line borne noise than the raw magnetic field. Unless the magnetic field is strong enough to attract stray tools and fasteners, don't worry about picture distortion on LCD's. You can experiment with this. I have a flashlight that can support its own weight and more. Holding this up to my LCD doesn't effect the image. You can explore AC fields by wrapping a current carrying extension cord around the monitor. Use multiple turns because your extension cord is not likely to carry 400 Amps. This wrap will couple more energy than a simple parallel conductor a few inches behind the TV.If you are worried, wrap the conduit with a piece of grounded copper flashing. This is much easier to find on a construction site than a large sheet of MuMetal. ( I have no idea what the inspector might think about this.)I agree that the big risk is from aggressive lag bolts.

You can tell who the old-timers are here. They are worried about magnetic fields disturbing the picture. Was true in CRT days with magnetic deflection, not so with flat panels.

The only concern now what days would be magnetic fields disturbing an HDMI cable that's not shielded so well, or some other signal cable that doesn't like the magnetic field nearby.

Would you been having this concern if it was a 60A, 100A or 200A sub panel ?

From the looking at the picture, that is at best a 200A sub panel (judging off the size of the panel and the conduit feeding it).

It doesn't seem that a 60A panel would necessarily contribute less interference than a 400A panel just from the amount of current. The difference would be that more interference-causing things can be plugged into the number of outlets coming from a 400A panel than from the smaller amount of outlets plugged into a 60A panel.

(I use the term outlets as representative. Of course all the outputs of a 400A panel don't go to outlets.)

buzz's comment about the strength of the magnetic field reminds me of this little hospital oopsie:









And of course there's this, too:

Conduit could cross to the right after dropping through the top plate, and may not require a new, longer feeder cable.

I don't know how effective shielding can be for that, or any, current.