Is the T-Mobile iPhone 6s from Apple.com Unlocked? If I buy the T-Mobile iPhone 6s from Apple for the full price (no contract), will it be carrier unlocked?
Yes - T-Mobile phones of all versions are factory unlocked. iPhone 6s/plus will be just like the iPhone 6/plus in that regard.

This year, you can buy phones unlocked on Apple's payment plan as well if you prefer that over paying full price.