I didn't read the whole thread, but my 870 express had an issue with the piece that holds the shells in the tube getting stuck in the outward position: Rack action, no shell released from tube. When this would happen I would have to empty the magazine from the front and use a punch/mallet type setup to get the part moving again in it's notch in the reciever....
That is part of the reason I got my Mossberg, which worked great until very recently... see my other thread!