It shouldn't. The official Nexus 7 cases are only for the wifi model, and it says so in their website.
"Tingues en compte que aquesta carcassa únicament és compatible amb els models de Nexus 7 només Wi-Fi." (My google is in catalan but it says -about their case, not the Poetic- "Take into account that this case is only for the wifi model")
Most likely yes. I do not have a Nexus 7 Wifi+3G but I've done a lot of research on the device. All indications are that it's the exact same form factor with the addition of the 3G radio inside (and just a little more weight).
I am fairly certain that the 3G/mobile data version has the same dimensions as the WiFi only. As I own the WiFi only version, I can not know to a certainty. Just to be safe, I would try to compare the dimensions of the two tablets to make sure. Hope that helps!