I am very familiar with the internals of the model 500 and how they work. I am also familiar with the usual suspect issues that the model has from time to time and their cures. All you need to do with a Mossy is to not have rearward pressure on the action and it will unlock; ie pull trigger while not holding forearm. They do not need the bolt or forearm to go forward. Take one and try it for yourself. Not all designs operate exactly the same way. If the OP had said Win. 1300 or old school Ithaca 37 I would have a different answer to his question but he said Mossberg 500.
I can unlock the bolt and hold all of mine upside down and they will not cycle open under their own weight. Both 12 gauge Mossy's of mine have quite a few rounds through them and don't come close to exhibiting the behavior of the OP's gun.
With that I am done discussing this.