How to identify when to try a "Merriamac Coup" in bridge?
It really depends. If you think declarer hasn't got enough tricks after your sacrifice of the K, and would absolutely need to establish dummy's suit, then yes, you could try it.

It is theoretically possible to execute the Merrimac on the opening lead, but the situation would probably be very rare. I haven't seen any, but I suppose given some time, we can cook up an example.

Yes, Merrimac can also be done on the closed hand, if you have a good enough read of the hand (which you should strive to, in any case).

And yes, using the Q to knock out the K can also be termed as a Merrimac Coup.

Not sure if I answered your question.