Can I leave and then come back later in the day under the same admission from earlier in the day?

Yes you can

Yup i think you can re enter

We lived in Boston near 4 years and brought kids there for countless times. Unless it changes its policy, I think you can go in and out on the same day by showing the stamp on your hand or the ticket receipt. No time limit.

I guess the limit is only two hours

I think so. We left outside for 10 min. and came back by showing the seal they put on our hands.