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.