Unlock GE JB550GJ - Manual Self-Clean Latch?

Hello,
I have a 25 years old oven and it was working great. The other day I ran the self-cleaning and the self-cleaning cycle was interrupted. After that, I could not unlock the latch to open the door. No error code is available on this oven but something is holding the manual latch and won’t release it. The manual handle is on the outside of the oven door, but I was told not to force the latch open as I could cause more problems.
The oven is a GE JB550GJ and the latch is a manual lock. You slide the latch handle from the left to the right to lock the oven door for self-clean. (I searched the Internet and most solutions for unlocking the latch are for automatic locking mechanism, not for manual latches.)
I powered the oven down for minutes then powered it on. I gently sliding the latch to the left but it won’t release. I tried to run another self-cleaning cycle, but it won’t start the self-cleaning now. Do you know how to release the manual door latch? My specific questions are:
(1) What is holding the manual latch and not letting go? Is there a way to unlock the latch? What are the specific steps required to reset this specific oven and to allow unlocking a manual latch?
(2) I have the user manual but it only talks about how to use the oven. I need a service manual to help me understand how the latch got stuck. I searched the Internet but I could not find a service manual for this old model. Do you know where I can download a copy?
(3) If I pull the oven out and access the back of the oven, is there an easy way to unlock the oven from the back? Do you think my old GE has a locking pawl inside the back panel which I can toggle to unlock the latch?
(4) I searched the Internet and found no parts relating to the locking mechanism. Do I need new parts to repair this problem after I get the door to open so it won’t happen again?
Thank you for your help.