Hi,
On the Internet Speed issue.
Try setting the app to 500kbps. (Messages > Preferences > Video section > Bandwidth drop down.)
This is actually fast enough for 4 way Video chats.
1-1 Video chat can get away with 128kbps - Messages will drop the pic size and the frame rate to do so which effects the smoothness of the video.
Screen Sharing in AIM and Jabber accounts also needs these speeds. ( I am not sure about the Screen Sharing App method iMessages uses but all use the Apple Remote Desktop engine so it can't be far off).
Limiting the Bandwidth also has some other positives:-
If the local service varies too much this can drop it below that disturbance. (Messages can Buffer to a limited extent but this is based on early top speeds of a 1500kbps connection and things have moved on from there)
If one end if on a much faster server and they both set this speed there is less likely hood of one end "overrunning" the other and saying the other end has not answered just because it is slower.
Airport Extreme.
It is a while since I delved in to the current layout of that device.
In Pics I do have the Port Mapping Protocol that is the best thing to Enable was in the NAT tab.

It does not need Settings for specific Ports after being Enabled.
It then allows the Computers and other devices to "tell" the Airport to allow the ports it wants to use.

9:12 pm Friday; April 3, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad