1) No. Only ST branded phones on the CDMA networks. The new LG Zip I think is on the Verizon network.
2) No 4G on ST. The AT&T and T-Mobile services have HSDPA+ which is like fake 4G. More like 3.5G.
3) AT&T and VZW have the best networks in the country. In some areas one is better than the other, in most places they're about the same.