Der Jaeger,
I agree completely with most of what you said, and it may be that the British did fit their Greener crossbolts tightly, however, I know from practical experience that if it is fitted too tightly, as in a tight slip fit, any dirt present with stop the third bolt from closing all the way. You can use the arguemant that we should keep our crossbolt guns meticulessly clean so that doesn't happen, but we all know that in real hunting situation, dust, dirt, oily gunk, and things like fine weed seeds, work their way into our gun actions. The only way to properly clean a crossbolt gun that gets hung up with accumulated crud, is to completely strip and detail clean it. That includes taking the blind crossbolt completely out of the gun and going in with Q-tips and or pipe cleaners and getting out the crud.
How many people do you know who are willing or able to do that? Most are not. Have you ever completely dis-assembled any of your crossbolt guns and given them a complete cleaning? I have completely dis-assembled several of these guns and you would not believe the built up gunk that works it's way in there. The point of all this is my claim that as time went on, gunmakers loosened up crossbolt tolerances because of problems with dirt getting in and binding up the gun's action.
It is the same reason that a .45 Colt automatic isn't tightly fitted like some of todays race guns and target pistols. If you are going to bet your life on a gun going off every time in dangerous situations, you want to make Damn Sure it goes bang when you pull the trigger, and then that it doesn't open by itself when the first shot is fired! This is one of the prime differences between a cross bolted gamegun and a double barreled rifle used against dangerous game.
Even Hide Bound old british gunmakers could change if they had to in order keep their clients alive to buy more guns. I do understand the reasoning behind the crossbolt helping to conquer the various forces that are at work in a double, but a slightly looser fitted crossbolt will do all that as well and not suffer from every little bit of dirt that gets into it. Closely fitted guns are a thing of beauty but I also know that they can be an extreme pain in the ass. I once owned a great German drilling that was fitted so tighly that it would not close on some type of snapcaps. I had to work down the rims just a little to even get the gun closed. Any degree of dirt or dust in it made it totally worthless. Did I enjoy hunting with it? Yes, but I would NOT want to stake my life on it's dependability.