How do I unscrew a Wilton cookie barrel from the press?

Add a dash of milk, but gradually, so as not to make the dough sticky.

Try letting the dough set at room temperature for 10-15 minutes before baking. This will soften the butter, which will then become even softer while baking, and should help them...

You would measure from the tip to the very beginning of the chamber. (This is the most common method of measuring barrel length.)

Yes, you should refrigerate your dough overnight, and then take it out of the refrigerator and put it in the cookie press right away.

You should hand-wash it. The dishwasher will not clean it thoroughly.