Check the Rooting for Dummies guide, it's great. There's a section for unrooting (i'm pretty sure).
a)
I think the only thing you need is the RUU, could be wrong though.
b)
I'm sure you will lose all your data, should be just like a new phone.
c)
You can try TB, but I think that needs root to restore, so unless you 're-root', you'll be SOL.