MySQL Daemon failed to start - Can't find file: './mysql/plugin.frm'
The answer maybe in the question. "Can't find file: './mysql/plugin.frm'"


Make sure that file exists and permissions are correct, which you did.
Next, SELinux may interfering (see this post)


You may test the SELinux theory by running the following:

setenforce 0
service mysql start


To permanently turn off SELinux, follow the steps here.