Not difficult at all -
1) Edward IV lives five or six more years and is succeeded by an adult Edward V.
2) Edward IV dies on schedule and Richard of Gloucester choses not to usurp the throne.
3) Richard III wins at Bosworth.
There is no reason why Henry Tudor will not make a bid in scenarios 1 and 2. A young king on the throne is as good a target as a usurper. After all, when it comes down to it all the Yorkists are usurpers.
On the matter of Bosworth Field, if Richard had been nicer to the Stanleys, they would been less lightly to stab him in the back. He would have then had no trouble defeating Henry Tudor's French jail fodder. In fact being nice to the Stanleys would undoubtedly help in scenarios 1 and 2 even if Henry raises a better class of army for a Bosworth Field in them.