Please explain to me how Luke's feelings about the temple effect the balance of the force in any way, shape, or form. He's just an old fart sitting around. Has been apparently for some time. If his conflicting emotions (about anything for that matter) is such an issue for the force (which he also apparently cut himself off from a while ago), why didn't it just send Yoda to smack some sense into him via lightning bolt or whatever right fucking then when it became an issue? Because Luke's whole conflicting emotions, and disappearing act is basically what threw the force all out of whack the second he decided he should sneak into Ben's room and contemplate turning him into swiss cheese for thinking naughty thoughts. The force was plenty content enough with Luke's absence, conflicting emotions, and general lack of caring, when it was just pumping all it's good guy juice into Rey, ignoring literally every other force sensitive person in the whole wide galaxy.

Also, the force sure lets one side (usually the evil one) do a whole lot of curb stomping before it decides it's time to send a message and get back to "balancing" shit.

I will go with Angelus, this balance story doesn't make sense when the dark side is so far ahead and ready to destroy whole planet systems. If Yoda is willing to intervene to give some sense To Luke, he can intervene to kill his ennemies.

Being a Force ghost was not about becoming neutral, it was about to continue to help.

Well Yoda is the most powerful Force user who ever lived so he probably could, but he just doesn't because I guess he feels like people should solve their own problems or he thinks Kylo Ren should be saved.

Because that wouldn’t make for a very entertaining movie.

I mean, it’s Star Wars. I love the franchise (except episode 1. And 2. And also 3.) but they’re all flawed in their own way and not Kent to be taken seriously. They’re action movies.

Luke's feelings about the temple affect the balance of the force because Luke's influence was largely pivotal to the conflict we saw in the movie. If he was unable to move forwards with his character arc and get past his feelings about the force and the past, the resistance may have died in that base. As for why it didn't happen earlier, perhaps there were other avenues of balance that were able to keep things from breaking before that point (evidenced by the fact that the rebellion survived to that point still around). Additionally, we know that Luke had closed himself off from the force. Rey's arrival seemed to have made him conflicted about that decision, allowing the force (aka Yoda) to work its way back in. One interesting tidbit is that Force ghosts require a strong presence of the force to serve as a beacon to allow them to home in on the location or person they are looking for. It is quite possible that Yoda couldn't actually find Luke until Rey showed up. 

Also, I think the reason that the Sith are often able to push things away from balance, is because the Jedi seek balance while the Sith do not. Because of that, the view of the Jedi as the "light" and the Sith as the "Dark" is a bit misleading. Balance is a fairly natural state of life and death in harmony. Perhaps it would be more apt to describe the Jedi as "harmony" and the Sith as "entropy". As such, when power shifts to the Jedi, the galaxy finds harmony between light and dark whereas when the Sith find power, the galaxy finds disorder and chaos. Perhaps it could be said that the "dark side" vs "light side" are tools which are used as opposed to states in which the Jedi/Sith exist in. The Sith use the power of the dark side to throw the galaxy out of balance, which the Jedi use the power of the light side to bring the galaxy back into balance.

That said, I am no expert in the Extended Universe (or I guess even Star Wars as a whole).

The only idea between becoming a Force Ghost is to continue helping your side, even beyond the death. It is not about "balance", and first we would have to know what is balance. If Jedi care about balance and not the Sith, then restauring the balance mean eliminating all the siths and dark side user and let the Jedi rules and guarding the balance. So Yoda should be ready to act, unleash his new power on a Snoke figure who is close to his former ennemy Sidious.

If balance mean having an equal number of Siths and Jedis, then a Force Ghost must be neutral, must not take any side which Yoda is clearly taking since he tries to motivate Luke to act. This does not add up with your analysis.

Kylo should kill Rey. She's had it too easy for too long.

Yes. The universe is already broken. Might as well have some crazy fun with it.

I think it makes more sense to say that Force Ghosts wish to continue working towards their goals of harmony after death, especially for someone such as Master Yoda. Additionally, I don't think it would be unreasonable to assume that such Jedi Masters would fall closer to enlightenment after death and by extension, become more in tune with the force.

I think the reasons why Yoda doesn't act to kill Snoke is, well, first of all, killing a tree is a bit of a different thing than killing a powerful force user. Second, I don't think that is his role as a steward of the force. His role is to serve as a spiritual guide, not as an assassin. I think also acting as an extension of the light, murder shouldn't be number one in his bag of tricks. That is why the light side so often seeks to build people up instead of striking others down and that is why both Luke and Rey have attempted to help those who have been corrupted instead of simply killing them. While obviously some Jedi do kill, killing in and of itself is a tool of the dark side. That is one of the reasons why I do not believe that the Jedi should be synonymous with the "light" and in many ways, it is a limit of the physical body. Losing your life largely removes the necessary evil of killing (or at least shifts it down in the priority list a bit).

To ask why a spiritual guide doesn't simply murder all of his enemies seems to be largely missing the point. In some ways, it is similar to questioning why God doesn't just blow up Judas. He has the power, and Judas is obviously acting in ways which serve counter to his goals, but instead, God largely seeks to council Jesus and try to build up the good instead of tearing down evil (something which is very clumsily stated by Rose in The Last Jedi). Star Wars has a lot of parallels with religion, so I do think the comparison makes sense. 

I completely agree with all of this.

Hence......don't make your enlightened, spiritual force ghost pop in and just blow some shit up while cackling like a lunatic. Like, if that's his role, just have him talk some damn sense into (non)Luke.