Having received the Ricardian Bulletin this morning I find there is an interesting article therein by Ian Mortimer on the subject of Henry IV's claim to the throne.
If I understand him correctly, Mortimer argues that Henry's claim was based on his descent from Henry III via both his father and his mother. He chose Henry III because Edward I, Edward III and Richard II had all purported to settle the crown in a way that didn't suit Henry's book. Notably, in the case of Richard II, by leaving the crown to Edmund of Langley. He says it has nowt to do with the legend that Edmund Crouchback was the elder brother of Edward I.
Well, we have it from Adam of Usk that the Crouchback legend was discussed, as he was one of the team of lawyers who examined and debunked it. So I don't think it can be wholly discounted.
I also can't imagine that Bolingbroke was overly bothered by what Edward I, Edward III and Richard II had or hadn't decreed about the succession, given that in late 1399 he was in a position to pretty much do as he liked.
Maybe the truth is that it was all left deliberately vague, with just a cloak of spurious legality over a very dubious claim. Though I still don't understand why Henry didn't just claim as Richard II's heir male - he was that beyond doubt. Even if we think of him as being under attainder (which he sort of was) that status would have been automatically reversed by his coronation.