Hi,

1. If you want to mail whenever the instance goes down then you will have
to write a procedure which will run continuously and read the content of
Alert.log file and whenever the entry is found then you can invoke a procedure
which will send mails.

2. If the archiver process fails the database will not go down. It will just stop archiving
and your database will hang until the files are archived.

HTH.
Vijay.