1) Yes, it's a "web interface" -- however, there are tons of applets that will allow you to monitor status via desktop, browser, etc. There are plenty of firefox extensions that allow you one-click download control from Newzbin. Trust me, it's much easier than you think.
2) It's as secure as you make it. You can either bind it to your local IP address making it only accessible locally (also via username/password auth) or you can optionally bind it to your public IP address (again, using username/password auth) or even take it a step further (as I and many others have) by securing it behind an Apache/Lighttpd/ngix proxy.
3) Yes, you can set the number of socket connections -- and it does support SSL. Many people use Giganews with SABnzbd+ -- it's not some hackjob usenet downloading tool -- look around on Google, there's an active development and support community spread amongst multiple locations.
FYI, hellaNZB has a "web interface" as well. Both SABnzbd+ and hellaNZB are written in Python.