Answering the first questions.
1: The DS has two wireless modes, a "custom-designed one" (which is supposed to be the technically-superior choice of the two), and what's commonly called "Wi-Fi", which is a kind of wireless broadband internet connection.
The custom-designed one
could be compatible with the GBA's wireless adaptors (we just don't know), but it already looks to be superior to the GBA's setup. The wireless GBA adaptors are supposed to be able to connect a maximum of
5 players, and I couldn't find any official numbers on it's range, but I saw some people saying "over 50 feet". The DS can supposedly connect 16 players, and has a guaranteed range of more than 100 feet, or 30 meters (there was some confusion at one point, because
Nintendo got mixed up and said it could connect over "30 feet").
In any case, the GBA's wireless adaptors only work with specially-designed GBA games (which so far means "Pokemon"), and you couldn't play DS games on a GBA, so the potential of Nintendo's two recent wireless types being compatible is very limited.
2: People are saying that the GBA's wireless adapters have rock-solid connections. I wouldn't know about Wi-Fi. So, you can make a guess about the reliability of the DS's connections. As for the range, it's 100 feet, but if you bounce that off the internet, then... who knows? But the farther your range over the internet, the more potential for lag you're gonna get. That's just a part of online games in general.
3: Use of the wireless modes will entirely come down to what features are used in individual games. Nintendo is most likely pushing the custom-wireless mode the same way they've always pushed the link cable. Some people have (perhaps pessimissticly) suggested that Nintendo's not pushing the Wi-Fi mode, but Wi-Fi is "a standard", and anyone who already knows how Wi-Fi works (not me), already knows how to make a Wi-Fi-enabled game on the DS. There ARE already reports of developers making games with Wi-Fi modes.
4: Voice chat would eat up the bandwidth. Open up the potential for laggy games. But that's the same thing the XBox faced. I should hope that voice chat would be really common feature in the offline "custom" wireless mode. It doesn't seem like it'd be needed (did anyone ever need voice chat to play the Four Swords?), but people need to remember that 100 feet is long enough to play against several of your neighbors (depending on where you live). If the broadband wireless connection is as powerful as they're suggesting, and they've included a microphone in the system anyways, why the heck not?
5: I doubt that any game will be packaged with the system, right off the launch. Unless we see some INSANE price cuts on the PSP to bring it almost in line with the DS. At which point, Nintendo probably would choose a software bundle over a price cut (unless they were pushed even farther).
However, the DS will have some sort of interface that we know nothing about. Some sort of menus, and ways to tell if there are other people nearby. Pictochat doesn't seem big enough to sell as a standalone game. But is very well suited to the DS. So, if Nintendo's smart, they'll probably incorporate it into the DS's built-in interface system somehow.