I'm providing links to previous discussions just for convenience. This is not to discourage further questions and debate -- please feel free to raise any other points that you like to.

I think /u/vonadler provides a concise overall summary in "why dident the Nazis fight the 400,000 stranded Brits in Dunkirk?" at https://www.reddit.com/r/AskHistorians/comments/6kfmhk/why_dident_the_nazis_fight_the_400000_stranded/

With some more details and a small bibliography, /u/threechance replied in "Why did Hitler not crush the British army when he had it surrounded and cut off at Dunkirk?" at https://www.reddit.com/r/AskHistorians/comments/6lds2b/why_did_hitler_not_crush_the_british_army_when_he/djtaq0b/ . As well, their reply below ("As for the Luftwaffe") touches on the Luftwaffe.

As for the "if they had" part, I think there is a reasonable response looking at short-term consequecnes and not going too far into alternate history: "How important for the UK was the evacuation from Dunkirk? Would the U.K. have been forced to surrender if it fell?" at https://www.reddit.com/r/AskHistorians/comments/4o4asq/how_important_for_the_uk_was_the_evacuation_from/ , with answers by NotAWittyFucker. I think the top reply (d49nz2m) and the "Would've impacted it?" reply (d49rq0r) are enough; there are a lot more replies.

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If you want much more reading about Dunkirk, at the time of the movie Dunkirk there was a megathread at https://www.reddit.com/r/AskHistorians/comments/6pg8vd/dunkirk_megathread_the_battle_of_dunkirk_and_the/ .

/u/coinsinmyrocket was the speaker on an AskHistorians podcast that covered this topic as well: https://www.reddit.com/r/AskHistorians/comments/8bzeut/the_askhistorians_podcast_109_dunkirk_the_dawn_of/