I've actually never seen a changing table for a child in an airplane lavatory. I can only assume lots of parents don't know of their existance.

Quite honestly I think the presence of a changing table ought to be included in the on-flight announcement schedule. Heck, they waste time telling us all to use the lav in our own cabin already. What's wrong with an announcement like this:
"To our passengers who are travelling with young children, please be aware that we have changing tables inside the lavatories on this flight. If you are not familiar with how they operate, please ask a flight attendant for assistance"

If there is no changing table, and if there is a long period of time (an hour plus) until arrival, I would guess there might be a case made for some discussion about alternatives. But only for #2.

And if the flight is within the hour on a no-changing table flight, or on it's final approach, no changing ought to be permitted. Again, a sample announcement:
"We are now on our initial approach to xxx. At this time please make sure your.... (etc, etc). If you are travelling with young children, we ask you to wait until you have deplaned to change their diapers".

I think difficult situations need to be addressed directly and firmly. Heck, as I said the subject of using the toilets in your cabin class has already been addressed. I don't think passengers would blink at an expanded announcement. That would also give the rest of us a chance to react if someone chose to disregard the "rules"

That said (don't you hate that expression?), people ought to know better. But they don't. Unfortunately.

Rita