The mantle size on the first one made me think female with eggs. The fact that she was actively running around makes me think post hatch female. In Trappers case (as with my other species of post hatch females), the mantle was not as firm looking though. More stretched out and wobbley. IME a healthy merc will turn dark red (not a sort-of-maybe dark but dark, dark) when released to the tank and will head for the LR. Spent animals seem to loose their ability to color more than other species and will come out during the day at the end of their lives.
FYI the dark spots that show so clearly on all the mercs are their gill hearts.
I believe Tom covers these with his warranty if you want to check on getting a second (shipping is not included).
If the big, ugly crab has a strong pincher, you can either disable or remove it. Many crabs will drop their claws if dangled by them. If you can get it to do this, it is better for the crab's survival should it not be eaten immediately. To disable it break off on section of the upper part of the claw (or remove the full upper section). It will live awhile like this but this method is more damaging to the animal.