1) Nagios is showing incoherent information between refreshes
you probably have two instances of nagios running. This shouldn't happen but it has always happened and is mainly due to the use of the "nagios restart" command. Try stopping nagios, wait a momne tand check if there are any processes still running "ps -ef | grep nagios" if there are use kill. start nagios again. If this fixes your problem and you used nagios restart, do NOT use restart again (reload has no problems AFAIK).
2) Why should i install from source if there are packages avilable?
Simply because the source install is "standard", while each package maintainer uses different paths depending on the distribution they are porting the software. From my experience updates are also quite slow in packages.
I'll try to keep this updated and expanded as required...