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For point 2, I think its for waterproofing. If there was a bolt all the way through the concrete water would find its way between the bolt and the concrete and break down both the steel and concrete resulting in ceiling leaks and structural problems for both the hangers and the tunnel roof. I know that on bridges, codes do not allow for bolts to extend through the deck, I assume the code for tunnels is similar.
From what I've heard and seen on the news, it seams like there was either a flaw with the anchor system design or just poor installation. The factor of safety is very large, I want to say around 4, on tension hangers like that so something has to be really wrong for them to fail. No epoxy definately is something really wrong.
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