"I wonder whether this is more a result of confusion (either the late Mr Collins's memory failing him towards the end of his days, or on the part of a proud relative attempting to decipher personal papers and getting entirely the wrong end of the stick -"
I've dabbled in Family History and seen the same effect - people's memories change and when they are related 2nd or 3rd hand they become quite unreliable - often a useful guide as to where to go and look but the actual dates and facts are often skewed badly