Trump is bound by no law to release those returns. Besides, you, me and the majority of Americans probably couldn't decipher them anyway. I do think he should release them just to shut Hillary up.
As for the campaign worker, it's important as to if she was hired/paid or a volunteer. She was paid and I believe she was directed to start that rumor. I think she was then released to make it look good and probably well rewarded.
When a judge directs a jury to "disregard verbal testimony", he knows that it's a wasted order. While it can't be a part of the deliberation, it is in the back of the minds of those jurors anyway. In this case, the worker made the statement and no apology kept it from spreading like wildfire which is exactly what the campaign wanted.
If you recall, that was a pretty bloody primary between Obama and Clinton despite the mutual ass kissing you see going on now.