Now about the Gemara.
You mentioned a Gemara earlier that seems to contradict the one you mentioned later. The first one says that if you are suffering you should check for sins or Bittul Torah, and as a last resort, Yissurim Shel Ahavah. The other Gemara says that we don’t know why good people suffer.
So, we must say, and it is obvious to learn, that the latter is about those few cases where everything was ruled out (including, perhaps, Yisurim Shel Ahavah), while the former is the general rule.
Now, the Gemara says that Moshe Rabbeinu asked for three things, one of which was to understand Tzaddik Vera Lo Rasha Vetov Lo. The Gemara first says that he was answered that only those who are children of Reshaim suffer. Then the Gemara says that the answer is that only a Tzaddik She’eino Gamur will suffer.
If you look at the Gemara you see that Reb Meir didn’t actually address the suffering of good people. He said that Moshe Rabbeinu didn’t get his answer on the issue of Tzaddik Vera Lo Rasha Vetov Lo because a Rasha can get completely undeserved goodness. As the Ramban explains in Shaar Hagemul, Hashem is good and will sometimes dole out undeserved goodness.
The Ramban seems to learn that this Gemara is the usual answer. Usually only a Tzaddik She’eino Gamur will suffer for those few sins that he has, as the Gemara explains in Keddushin. However, says the Ramban, Sefer Iyuv is about even a Tzaddik Gamur suffering.
According to this Ramban, both Gemaros are in league. They are both saying that all, or almost all, suffering is from sin, however few and small. The very uncommon completely undeserved suffering, without any sin whatsoever, is what Sefer Iyuv is addressing. And Elihu’s answer is Gilgul according to the Ramban (and any other Mekubal), or Malach Meilitz mentioned in Shabbos 32 according to the Rambam IIReadC.
Others learn that the Gemara is hinting to Gilgul by referring to the Tzaddik with baggage as Eino Gamur. This satisfied the Derech of the Rama Mifano, quoted and promoted by the Shela Hakadosh, that the Maskana doesn’t really throw away the Havva Amina. In this case, Tzaddik ben Rasha refers to his previous Gilgul.
Interestingly, Iyuv himself has a reason for his suffering according to the Gemara. He was quiet by Paroh when they were discussing the Jews. He didn’t speak up up in their benefit. So what is his question? Perhaps that was in a previous Gilgul. I have another Pshat as well, though.