I don't think you've made a mistake ordering the WeatherLinkIP. As you stated, it's a total hands-off way to get data online easily. I have 3 Davis stations, 2 with dataloggers linked to pc's and 1 IP. I wish all three were WeatherlinkIP.
1. Yes, you will be using Weatherlink 2.0 when you get everything hooked up.
2. I believe your understanding is correct. I'm not sure if the $47/year PRO plan would still have to be purchased from Davis since you bought the IP. That may be a question for Davis.
3. The update rate for Wunderground and CWOP is 15 mins. The 5, 15 and 60 minute option you mentioned is for the EnviroMonitor which is a cellular update option, but does not affect the Weatherlink 2.0 updates to WU and CWOP.
4. You will be able to configure everything on the web with your Mac for initial setup. Updates to WL 2.0 are every 60 seconds by default, so there is nothing you will need to configure for this.
5. I've never owned a Meteobridge, so hopefully someone else can answer this for you.
Enjoy your purchase!
PS: You can see my weather stations running on WL 2.0 at:
http://shoalsweather.net/fhs and
http://shoalsweather.net/northflorence