nagios is robust enough to handle what you request and much more, but you're missing how it works. Nobody has responded because these answers need to be understood by reading the FAQ and documentation at nagios.org
Some short answers to your questions:
Nagios is not really designed to read from an existing database full of syslog and snmp alert information. It is designed to run scripts, which return an exit code (0 = OK , 2 = CRITICAL) and then perform an action based on that result. If you're gonna cut over to nagios, you're going to want to set up your SNMP traps in your nagios service definitions.
I use syslog-ng as my syslog daemon, and it integrates into nagios very well. Check out the manual here, it has its own nagios section:
balabit.com/dl/guides/syslog ... min-en.pdf
Check out screenshots of nagios for what you can display. It uses cgi pages which construct various pages of info. I reccommend grabbing a prettifying skin/CSS package for it called Nuvola ( nagiosexchange.org/Image_Pac ... iew%5D=252)
Nuvola uses a bunch of css and new graphics to make your interface look waaay better. We've got that skin on, and we've got a couple 46" lcd's displaying nagios and graph info in our office