For first question, I think it's NO.. airport can't connect to multiple networks at the same time.
For 2nd, yes, you can connect to both Ethernet-based network and wireless network. but IMOH, ethernet network will be the first priority over the wireless because of its transfer speed.
For file sharing, of course, you can share.
go to finder, and right click above the Finder Toolbar
and choose Customize Toolbar. Than you'll find out "Connect" and Drag it to the Toolbar. Then, you can quickly access to the network easily . just try to access with ip (for example) "smb://192.168.1.131"
You can browse files easily in this way.