As you have discovered, the members of this forum really do like to help their fellow costumers!
I agree, try to make the shirt more form-fitting if at all possible. Maybe it just needs to be taken in a bit all along the side seams and under the arms for the sleeves? To keep the shirt tucked in, you could use buttons, snaps or velcro to attach it to your leggings. My "shirt" is actually a body suit made from a stretchy fabric to keep it from puckering or pulling out of the skirt as I move around.
For the collar or neck piece, mine is actually sewn to the shirt along the outside edge. I'm not sure what the other ladies have done, but this worked best for me. Whatever you decide, you will want to use something that runs continuously all along the outer edge so that you don't end up with the shirt fabric pulling strangely in only a few places because that's where the velcro is. Honestly, I found the collar to be the most difficult part to make, and fit correctly.