In the photo below of the top of my main panel, wire A is clearly not in a good spot. Perspective in the photo is distorted, it's actually about 1-1/2" from the mains, but still ... Anyway, at the left, not visible, it goes to a ground screw in the panel enclosure. If the incoming service neutral is bonded to the enclosure some other way then wire A is clearly unnecessary -- and even if needed it doesn't need to go across because both sets of bars are connected to the incoming neutral, and hence to each other.
Any good way to find out if the enclosure is bonded some other way, other than shutting things down, removing the neutrals / grounds (they need to be neatened up anyway), and looking for a bonding screw? Would it be normal to have an older main panel where bonding of the enclosure was not built in?
If the enclosure is not bonded then it clearly should be but I'm thinking a piece of bare #12 is not the way to do it. I couldn't figure out what size wire is required by code for bonding a main panel enclosure. Does it follow the EGC bonding table in 250.66, which would require #4?