The knife trick sounds good and would work in some instances BUT most of the time a cartridge tray is half way stuck into the laser section. If you were to try and force the tray out you WILL damage the internal parts. Not recommended. The problem is an internal one and just removing the cartridge will not solve the problem. It may work for a little bit but it WILL do it again. Service is required to solve the problem. I can help in that department.
As for the ML changer problem: You may try and disconnect the Fiber Optic cable at the changer along with the Power plug. Then reconnect the power cable and then the Fiber Optic cable. Then press the eject button on the changer BEFORE inserting the cartridge. This may solve the problem.
Cleeve Morrison
La Jolla Audio Repair
www.lajollaaudiorepair.com