Yes, the B12 absorption is problematic. Still it are 75% of vegans that have B12 problems, not 75% of meat eaters ;)
Regarding Fukushima I am not so sure. It depends wher the stuff gets transported and you get the nori from.
For the average ocean the bomb tests on the bikini islands etc. should provide more radiation then Fukushima.
btw. here in Germany it is still warned to eat more then once per week of fungi you got out from the woods. Some of them are still XX times more radioactive then the max. allowed (for stores) amount because of Chernobyl.
I don't know how good nori is in absorbing radioactive materials, but the shiitake are probably more dangerous.
Any figures produced for B12 need to be carefully sourced because, as stated, there is disagreement and claims being made by qualified individuals that measurements from older date is flawed. Here's a page that contradicts totally the 'accepted wisdom': http://www.vibrancyuk.com/B12.html
I don't know enough about the radiation figures involved to compare fukushima to any other event, I just know that given a choice of eating radioactive things or not - I will choose not!