Once a flight has been credited to a program, nothing is going to happen until that credit is reversed. So you will need to wait until Amtrak does that -- and getting a program to reverse credit has always been difficult. If they do get reversed, then your friends need to ask SQ for missing mileage credit.

And yes you can credit UA flights to the Amtrak Guest Rewards program

As you heard, the name mis-match should have prevented the credit in the first, so that is odd.