You don't want to be a root user here; that's asking for trouble.
To deal with the first problem first, you can set the suid bit of /usr/sbin/pppd by running just this one command as root:
chmod a+s /usr/sbin/pppd
Once that's done, any user on your computer will be able to use pppd to connect to the internet.
The pppd is probably dying because it detects the fact that it's been started as root, and so cannot start up safely.
As to writing /etc/fstab; what are you doing to get this message? When you save a file you have to have sufficient permissions to access it; root can change file permissions but does not automatically get full permission to every file. Imagine the carnage you could cause if you tried to write to /dev/dsp (the microphone). Root can also not write to a read-only partition.
What is the output of the commands
ls /etc/fstab -l