The way I read them, I assumed that if they didn't marry until after #5 was born, then they'd have five bastard eignes. Though both definitions do state that it's only the *eldest* son.
I think, though, in your supposition, that son #1 would still be a bastard eigne, according to both definitions.
I also think it very interesting that daughters don't come into this anywhere at all. I know that was the case for many centuries, that bastard daughters were rarely acknowledged. I think in the case of sons, they would have been more dangerous to ignore and/or more useful to acknowledge.