This post is provided as-is, use at your own risk.
My latest lab is on Server 2008 R2 and I was having a weird issue with the MP not installing. Even though I had configured WebDav correctly as per the TechNet article here, however my mpsetup.log was still giving me errors about WebDav not being correctly configured. Upon opening up the webdav_schema.xml I could see that it wasn’t actually set correctly.
I tried clearing the settings, uninstalling WebDav, restarting the server and reinstalling, reconfiguring, but even that didn’t work.
This post here pointed me in the right direction and credit is due where credit is due!
After manually modifying the webdav_schema.xml, then I was able to successfully install the MP site role.