1. Jackson should have stuck to the original story and knocked it down to two shorter movies rather than three over-bloated ones. The Gandalf side-plot with the Necromancer makes a really good 45 min fan-film (The Battle of Dol Guldur) but was just bloat in the context of these films.
Once Del Toro left the production, and Jackson came on to direct just to save the money spent on development, the studio demanded it be a trilogy of three-hour films, just as LOTR had been. Jackson had little choice but to make it that way, and as such, he and his writing partners, Fran Walsh and Philippa Boyens, went out of their way to make whatever they added be as close to something Tolkien would have written as possible. Jackson relied on the other two being huge fans of Tolkien to achieve this.
As such, the story of Tauriel and Kili bears more than a passing resemblance to Tolkienesque romances, including but not limited to "The Lay of Luthien".
2. There was a fair opportunity for Legolas cameos: once in Mirkwood, and at his father's side in the Battle of the Five Armies. Cameos, not entire subplots.
Once Jackson was required to add four hours of material to what should by rights be either one five-hour, or two two-and-a-half hour films, cameos went off the table in favor of extended subplots.
3. I think Tauriel could have been worked in as well, but not as a main character. Any time you had the elves, she could have made an appearance. The bottom line is that there were just no female characters in The Hobbit as written by Tolkien. Not every movie or story has to have a female hero or character. In others they play the central role.
Every production has one limitation it shares with all others, regardless of source material, story, or production staff; it must appease a general audience. There is no such thing, if there ever was, as making a big budget small film. For a film to make money, it must draw audiences, and anything that can cause word of mouth to negatively affect the box office must be reworked into an asset, or dropped.
In today's world, that means appealing to a plurality audience, and that means including anything they can to draw the diverse elements of it into the theater. If that means adding a female character into a story sourced from a 'boys' club' style story, then you add a female character. Especially if not doing so causes the 'girls' club' to attack your production. It doesn't have anything to do with what the original was like. It has only to do with how best to appease an audience that either has no familiarity with, or actually doesn't like, the original, so that you get the largest audience possible.
