Never was it stated that Herriman had grown a mouth inside of his chest, or a harpoon for that matter. Also I don't think the neck guard should be able to absorb all that energy and all that happens is it knocks you backwards. The spikes stopping their momentum is fine, but remember your still getting ran into at that speed so it isn't going to leave him unfazed. 

also there is now way Herriman had planned on Mifune doing what he did seeing as how he was trying to impale him. Lets also not forget that in your last post you turned at a forty five degree angle so it isn't hitting you square in the chest more in your side. Thinking about it now Mifune running into you sideways would make him fall forward and you fly off your feet sideways not fall backwards on spikes.