With regard to your first question about the Avaya -
There are 3 elements involved, the phone, the GDS and the IP-Office. Each must be compatible with the other in order to communicate to and from one another with the IP-Office acting as the mediator in between. If the IP Office cannot handle the media streams needed to relay from the GDS to a phone, then direct IP dialing is the only option as shown in the manual as this will by-pass the IP-Office.
The GDS spec sheet indicate that m-jpeg and h.264 are supported.
The IP-Office must still support the video formats or at least allow media stream re-direction so that the media flows directly between the GDS and GXP. If the IP-Office will setup the call between the devices and then allow the devices to re-INVITE between themselves, then it may work; hence why I suggested getting to an Avaya specific forum.
The bottom line is that you want to get video from a door to a phone and if you want to use the IP-Office:
a) the IP-Office must accommodate and pass the needed codec or format thru it to the phone, or
b) the IP_Office cannot accommodate the codec/media format, then it must allow media stream
redirection so that the two endpoints communicate media streams directly between one another
If neither of the above is an option, then you will need to use direct IP dialing as shown in the manual. I cannot say about more than one IP. Marcin has better insight into this area.
If video is that important, then you have two options for a more seamless transition:
1, See what Avaya has that can accommodate the need
2. Change the IP-Office for a UCM and then convert/re-provision the existing Avaya phones and manually provision.