If I read the Missouri law correctly, if Akin withdraws, MaCaskill runs unopposed. On the other hand, if he waits three more weeks, they can replace him on the ballot if this does not blow over.
http://www.freerepublic.com/focus/news/2921944/posts?page=1
Regarding replacing Todd Akin if he withdraws - the Republican Central Committee will choose the replacement. That replacement can be someone who lost in the primary. So Steelman or Brunner could still end up as the candidate. Akin would have to withdraw and get a court order by Sept. 25.