Yeah, I don't think they will bug someone using 10GB, but I could see 25 or 50+
Not really. Only a small percentage of t-mobile customers are on unlimited. And of those the vast majority use less than 10-20GB a month. And even less use the hotspot often, and even less use it...
LG 236c at WalMart is a nice flip phone, and only $15. http://www.walmart.com/search/?query=lg%20236c
I'm sure more than 3,000 have passed it. They are only concerned with the 3,000 most egregious users which is great for the vast millions of T-Mobile customers that will be left alone.
It should work fine. If you're on a 4G phone, you won't be able to do it.