Hi Zachary,
I had deleted the previous 'arcgis' folder and Virtual Dir in IIS - you get a warning saying it already exists when trying to run the WA install.
I'd seem similar posts regarding using the command line to install if there are multiple web sites. Tried that and still failed (plus yesterday I only had 1 site in IIS). I was able to fix that 2878 error .... issue was due to the bindings in the website - there were 2 entries pointing to port 80. I removed the duplicate entry from the bindings, restart the website and off the races with the WA install.
Still cannot get the WA to configure though. After it installs when I go to <server>/arcgis/webadaptor and enter the server:6080 (or FQDN), Site Admin user & pwd I get this error -
"Additional non-parsable characters are at the end of the string"
If I change the pwd to be only alpha-numeric (got the idea from a known bug involving SSL and pwd with special chars), I get a different error -
"Could not find any recognizable digits"
Waiting on Tech Support for next wall to bang head on. Ugh.