I figure you've tried everything shave related under the sun... but I'm going to ask anyways since my neck is uber sensitive and not anymore.
Pre-shave oil??? alum block??? cold water rinse??? all things I have just started using... and all have helped produce 0 irritation. I know it aint the razor cause my technique can't be that great after 7 or 8 shaves but heh, my necks been thanking me.
Also find waiting a bit before applying ASB helps... as the moment i slap that stuff on any redness follows... where if i wait till say before bed and put it on. I wake up to a clean neck.