I think I would run a dedicated common ground just for the coil pack grounds. You could terminate the common ground onto a hard chassis ground or even run it back to the battery common. A nice small strand #12 or #10 (overkill) would work nicely.
Grounding to the engine, while commonly done is often a source of problems due to engine/chassis ground problems. Of course if your using solid engine mounts with no rubber it would be fine.
No. Run a wire from the under hood battery ground bolt then route it close to the coils, then connect each coil ground to that wire. You could even shield if so inclined but I'm not sure a ground would be worth shielding.
Your schematic says Pin C must ground to the head the spark plugs sits in, so just terminate the other end of the ground to the head.
My battery is also in the trunk, but there is still that big bolt on the left fender below the fuse box that was used for the battery ground. Failing that, you could use the ground point right behind the head, the one the block uses as a ground. Run those grounds to it in parallel.
I'm sure you could put the head ground under a intake manifold bolt or something like that, to the same effect.
