1. Yes. You'll need a GSM X1 SIM.
2. You should be able to transfer everything over by using ting.com/activate. It should ask if you'd like to use a GSM SIM card, and if you'd like to bring your existing Ting phone number. You can keep your Ting number. In the unlikely event you have trouble with it, or it seems like it's not going to let you keep your number, send us an email at email@example.com or chat with us at ting.com/support.
3. Yes. You absolutely need to know beforehand if the GSM network is going to work for you. As the networks aren't identical, you'll want to check the GSM map on ting.com/coverage. Because the 6T isn't CDMA compatible, if the GSM network doesn't work out for you, you'll have to go back to the G4 or pick a device that is CDMA network (read: Sprint) compatible. Be prepared for this possibility.
But if it works out for you, it looks like a great phone!