1. The Airport Extreme is, with the last updates, in compliance with the 802.11G standard.
2. Can't say for sure, but i recon it will work, since DLink also claims to follow the standard. I use a Linksys WAP54G access point, and that works fine with both my Windows computers and my PB17.
3. It'll pop up as available and it'll even connect automatically if you've set airport to do so. You might have to configure access list (MAC address) on your access point if you're using that as a security feature.
4. Well, I can again only speak for the Linksys equipment, but it works fine on 54Mbps there. Since both DLink and Airport Extreme claims to follow the standard, I can think of no reason why this shouldn't work.