I have fresh installed 4.5.1 from the install package and from the source package.  Neither contains the necessary provider directory in the correct place (according to other posts) for the authentication module to function.  Indeed the install package does not include it at all.  I have tried to get autologin to function by:
entering proper AD entries on authentication page (confirmed)
removing anonymous access from windowssignin.aspx
placing AuthenticationProviders directory from source package

Site will only authenticate against AD if credentials are entered manually:  No auto login.  
This is not a surprise as I have never been able to get site to autologin in mixed mode without my workaround code.  I have also tried removing anonymous access from entire site.  To my knowledge this is the only difference between mixed mode and not mixed mode.  I also tried entering a username and password to "<identity impersonate="true"..."

I have reset iis and rebooted the web server.  Cleared the cache of the site.   Tried all things found on "tatertots" page as well as others.  Added "ADFix" dlls and setup file.  Recompiled core.  Recompiled relevant modules.  Each time testing each change for success.  I think I have reinstalled DNN 4.5.1 several dozen times in the last two days.  Still the windows authentication login dialog appears and user must log in using AD credentials manually.  All this on a server that is happily running a 03.03.05 version with over one hundred child portals in mixed mode with autologin functioning using my mod.

Maybe I am missing the "comprehensive setup procedure" for this rather complex setup?  If I could get it to function, I would write it. Is "taterTots" still the most comprehensive?

I fell for that one also for a short time...  There are references (in the web.config) to the authentication files residing in a certain location that doesn't exist.  When I looked into it deeper, I found that there were a LOT of files (in that same area of the web.config) referenced in folders that didn't exist, not just the authentication files.  When I dug some more, I realized that this has been the case for many versions of DNN, and that since it has been running fine up until now this was probably the "norm" and nothing to be concerned about.  I don't know why those references are there, as I didn't dig into it any deeper, but apparently it's not a problem.