I think iTunes works like a hard disk. Sharing librarie is the way to get media to differents macs. So, I think it will work on AppleTV. Each AppleTV will get independently the sources.
That should work. Just don't expect to "stream" videos to 2 ATVs at the
same time, that requires lots of bandwidth.
It depends of the router. The most popular routers are defined by the 802.11b and 802.11g protocols. It operates at a maximum bit rate of 54 Mbit/s. The 802.11n up to 600 Mbit/s.