To the first part, I think Zoro and Luffy were put together for a reason, to take on the big players, plus Zoro mentioned not getting too injured as well, his most underrated skill Post Time Skip has been his critical thinking
Zoro's past has yet to be revealed, i found it interesting that Yasu took immediate interest in high praise for Zoro and has never given anyone else that praise before, he also knew franky law and usopp were his pals. I think it has to do with either the Wado or Shusui, but he definitely saw something in zoro skill and strength wise