Newbie Question on 2016 F150 2.7L SuperCab 3.55 4x4

First question, yes 5100 lbs. Max
2. Yes, putting it in the TT will only add a small percentage to tongue weight.
3. No, you can tow up to 100% of your vehicle ratings as long as you don't exceed any. Why would they give you a max rating then tell you you're only supposed to tow at 80% of it?