John of Eltham, Earl of Cornwall

John of Eltham, brother of Edward III, king of England, lives longer, marries, and has several children. Could his heirs attempt to seize the throne from Richard II, instead of Henry of Lancaster? Or could a civil war break out between John's heirs and those of Edward III?
 
While we'd need to see descriptions of the lives and personalities of these descendants to make real comments, I think you may have butterflied the Wars of the Roses away altogether. If, when Richard II goes mad, these folks are faster on the draw than Henry they could certainly be the successful usurpers - or declare themselves Regent instead of usurping outright, since if I read the geneaology tables correctly Richard II's proper heir was Edmund Mortimer, 5th Earl of March (a 9-year-old boy if Richard leaves on schedule).
 
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