The cart looks fine. I’m curious, do you know for sure that you will need a new cell sled? If the one you have is a 4G Alarm.com module then you should be able to reuse it with our service. If it’s 3G then you will need a new one.
Yes, you can use the 2GIG takeover switches and your current sensor setup will work fine with our service. Nice wiring, by the way.
Regarding setting up your GC3 and smartthings, have you seen these topics?