Hook,
I'm going to forego the debate and answer your root questions
1: Correct, you'd have to disconnect from the Skyguard network or disable wifi on that tablet to get it to update from the internet
2: Also correct, assuming you have "Attempt internet weather download while in flight" selected (in Menu -> Setup)
3: Possibly, but if you have the space, I'd recommend having both tablets up. You can turn the wifi off on the tablet that has cellular and it will update via the internet. Then you can leave the Skyguard on and let the other tablet display the traffic and weather from ADS-B.
As for your followup questions: iFly will update weather over the internet as our server gets new data from NOAA, about every 10 minutes. The download includes the whole country and is only about 500kB, so you're looking about 3-5 MB per hour of data for weather. That doesn't include some things, like high-res winds aloft, which are updated every few hours.