[B. Dudley]

If you search “unhooking” on this board, you will find the topic is a common one to come up from time to time.

Try dry firing and then removing the barrels. That is a temp solution to the problem. If they still will not unhook, then a smith familiar with Parkers is in order.
The forend is a head scratcher kind of based on your description. Bear in mind that the spring in the forend are very strong. So you should not be able to move it by hand. Will the forend install and stay on?