I'm sorry for how this is turning out, and am sure you are frustrated.
1. IF the chambers are indeed 3" (and not so marked on the barrel flats) they have been lengthened and it is quite possible that the wall thickness is inadequate at the end of the chambers. A smith with the proper tools could easily measure the chambers, and wall thickness.
2. The crack needs to be repaired; it will only get worse with use making repair much more difficult, esp. if the stock splits. And there are likely more cracks in the head of the stock.
3. The triggers need to be pushed down/forward against very slight spring pressure while installing the locks and the connecting screw.

Bottom line is that a Smith expert needs to evaluate your barrels and repair the crack, which will likely require shipping the gun. Post your email and someone might be able give you a gunsmith's contact information.