1-- The difference is that the nouns in the first sentence are plural. The article is not always necessary, and here it is optional. Why does it not read the medieval religious leaders? They too must be a limited group. The article with plural nouns seems to require real specificity: the kings and princes who are giving me a knighthood.
2-- I see no difference-- the article is again optional. (Sorry, Peter, I cannot even agree with your intuition.)