All this very good stuff leaves a question. If blunderbusses were not made for all the above listed reasons can any one suggest a more plausable reason for a flared muzzle?
Ease of loading.
And maybe to make the enemy you was shooting towards think you had a larger caliber than you actually had?
Another point in the "Nope, they didn't shoot nails, glass, rocks ..." argument
Early blunderbuss (and cannon) had brass or bronze barrels. Much softer and more prone to damage and failure than a steel barrel.
The nails, glass, and possibly rocks are harder than the brass or bronze. It would be possible to destroy the barrel (gouging it out) in just one or two shots, causing catastrophic failure by the third or fourth shot.
Of these the glass is harder than a steel barrel. (You can use ceramics/glass to sharpen any metal knife. Using any metal knife on a ceramic/glass plate
will quickly dull it - even if it has one of the modern "super steel" blades.)
Depending on the heat treat, the nails might be harder than a steel barrel, too.
Shooting other than a lead ball (or lead bird shot with the proper over-powder and over shot wads) out of a steel barrel would be just as bad an idea as shooting those things from a brass or bronze barrel.