Looks to me like Jackson actually backfilled the Hobbit with material from LOTR, both the Appendices and the main story—after all, Gandalf’s story as to where he went was expanded in LOTR, including finding Thrain in Dol Guldur (something that is in the Desolation of Smaug extended edition). Rewriting Azog as a main character turns it into “alternate universe” and really ought to be left to the fan-fiction sphere, but it seems (at least to me) that Jackson wanted to evoke the atmosphere of the Fellowship being hunted by the Nazgûl.
As for the “sweetness” of The Hobbit, even Tolkein expressed regret that the story was not more LOTR-like, and had to rewrite Gollum’s part in the “Riddles In The Dark” chapter to match what was going on in the later story.
The Fellowship actually started out very Hobbit-like, which was part of its power (over me, anyway) - it was all nice and sweet and avuncular until the Nazgul showed up in the Shire. At which point I had to pick my jaw up off the floor. It was sort of like paddling around in the shallow end of the pool thinking you’re safe and all of a sudden realizing that you were in the deep end all along. Problem is, you can only do that once, but it sure worked on me.