To me it looked like he misjudged the ball pretty early on (or it was moving oddly). After he calls fair catch he goes up to the 17, then retreats to the 15, then moves over about 2 yards to the hashes, then dives to the 17 to try and catch the ball. He never did get under it. I doubt the gunner had any influence on this part because most of it happened well before the gunner got anywhere near JT. The gunner only had an impact because JT misjudged it and tried to dive to catch it. Maybe it should have been called interference, maybe not, but the more important thing, IMO, is JT learns don't dive for punts, and hopefully the midjudgement is not a regular thing. FWIW I think he'll be a fine punt return and will learn from that experience.
I guess he should have stepped up into the spot and taken the contact and hope the refs call it right?
That's exactly what he should have done. As a returner, once you call fair catch, you have to focus 100% on catching the ball. Any contact will be a flag. If JT allowed the defense to make him not go to where the ball was landing then he made a mistake.