even if Richard was dead, Henry wouldn't have been the heir - Elizabeth would have been. And I agree that a woman ruler would have been rather unbelievable at the time, however whoever she married would have been a strong contender.
So, maybe Henry was generous because he knew that even if he did everything he could to make sure that everyone knew that HE was the monarch, not his wife, he should stay on his wife's good side (or her family's for that matter) because in the end he wouldn't have been half as strong without her presence at his side.
I doubt Elizabeth Woodville would have supported Henry'ss claim instead of her son's... no matter how generous he was.
And, on the other hand, Henry was really not that generous to Elizabeth Woodville - she died in total obscurity.