1. Band 30 is useless at the moment, don't even bother yourself about it. Everyone on reddit is getting themselves worked up trying to find a way to get an AT&T iPhone for that Band 30 and it's all for nothing. Fuck Band 30.
2. All models of the iPhone 6S can accept any SIM and have the hardware to run on any network worldwide, GSM or CDMA. As long as the carrier has unlocked the phone, any SIM in the world will work
3. If you buy it full priced from AT&T or an Apple Store with AT&T activation, it will be unlocked or can be unlocked. If you buy it on a 2 year contract or with the NEXT payment plan, it can't be unlocked until the contract is fulfilled. However, you can only buy AT&T phones if you have an active AT&T account
4. I'm dead serious, fuck Band 30. It's not bad, but unless you are in very specific areas it hasn't had a wide rollout yet. Buy whatever phone fits your needs and your plan, I swear you'll be okay