Boilers

Hi,
Could someone talk about boilers a little bit? I gather that you need at least two or is it three rows of rivets on a boiler seam to be legal?
It seems that in order to run the WW&F 9 the museum will have to get a new boiler built. How then was a "poorly designed boiler" run over three or four railroads for over 40 years without problems?
Did boiler inspectors and shop men really know that much more than they do now? How and when did such a boiler become illegal? Just age?
How often did an engine get a whole new boiler? I assume that it would be during a major rebuild? When the D&RGW converted those 490s engines from standard gauge to narrow gauge did they get new boilers too?
Lastly, is it better to retire an engine to museum display or replace such a large historic part so it can run again? Thanks Ted.