It took a couple hours for my port to complete from AT&T to T-Mobile. Transfer was very straightforward, and required no contact with AT&T. I did have to bring a copy of my last bill, and give my AT&T account PIN so yah the transfer could be approved.
The only hassle is that I had to deal with carrier store reps in getting set up, but that's par for the course regardless of carrier.
As for the ETF reimbursement: You do have to hand in *A* phone. It doesn't have to be the smartphone you're using though. I found an old AT&T Nokia dumb phone in my kitchen drawer, didn't even have the charger. Handed that in and it counted.
You also do have to buy a phone from them. But this too, can be the cheapest, dumbest phone they offer. At the time I did it was some Pantech model that full price cost $70, which can probably be flipped on eBay for about $30 or so. As for the remainder, you can probably cover that by having someone refer you, to get $25 back, plus unlimited data for free for a year (feel free to PM me if you need someone to refer you).