The IRS isn't into balancing acts so don't plan on them offsetting those years.  

I don't know why you couldn't send them all in one bundle.  But I have a fear of a very dedicated IRS employee grabbing the one on top and ignoring all of the other years so I would tend to mail them individually.  But that's just me and my paranoia dealing with the IRS.  It could just be all of those anvils talking.