(a) The Gemara gives examples of how marying a daughter of a Kohen is good and beneficial. It then gives two "maaseh listor" stories, apparently contradictory examples , in which R' Yehoshua becomes ill & blames it on his marriage to a bas kohen and R' Kahane has to run away. Which is true? Is it bad for a bas Kohen to marry a ben Yisrael also?
(b) Also was R' Kahane a kohen himself (Kahane=Kohen?) or not? If so what can we learn from the story?
(c) In addition the Gemara calls his going to Israel from Bavel "galut." That seems surprising. We always think of going to Israel as Aliya from anywhere and galut as anywhere but Israel.
(a) The Gemara first says, in the name of Rebbi Yochanan, that it is not good for a Yisrael to marry a Bas Kohen. The Gemara then challenges this from a different statement of Rebbi Yochanan in which he said that it *is* good to marry a Bas Kohen. The Gemara answers that the first statement was said with regard to an Am ha'Aretz, who should not marry a Bas Kohen, and the second statement was said with regard to a Talmid Chacham, for whom it is good to marry a Bas Kohen.
The Gemara then brings *four* incidents of a Yisrael who married a Bas
Kohen. The first incident involves Rebbi Yehoshua, who became sick after he married a Bas Kohen, and said, "Is it not favorable to Aharon ha'Kohen that his descendant cling to someone like me [a Talmid Chacham]?" This incident does not illustrate that it is bad to marry a Bas Kohen. On the contrary, Rebbi Yehoshua was perplexed why he became ill, if it is good for a Talmid Chacham to marry a Bas Kohen.
The second and third incidents, involving Rav Idi bar Avin and Rav Papa, respectively, make it clear that it *is* good to marry a Bas Kohen (Rav Idi had sons who became great Talmidei Chachamim, and Rav Papa became very wealthy).
The fourth incident involves Rav Kahana, who said, "Had I not married a Bas Kohen, I would not have gone to Galus." This implies that it is not good to marry a Bas Kohen. However, his students themselves pointed out that he was sent to a Makom Torah. He replied that he was forced there, and thus it is not good. The MAHARSHA explains that Rav Kahana, in his humility, did not acknowledge that he was a Talmid Chacham, and therefore he attributed his misfortune to the fact that he married a Bas Kohen. His students, though, knew he was a Talmid Chacham, and therefore they pointed out that his "misfortune" was really a good thing.
(b) TOSFOS (on Amud Beis, DH Amar) asks your question. Here, Tosfos concludes that Rav Kahana was *not* a Kohen, as our Gemara implies. Tosfos in Kidushin (8a) and Tosfos in Chulin (132a) assert that there were two people named Rav Kahana, one who was a Kohen and one who was not a Kohen.
(c) You are correct that we usually use the term "Galus" to refer to leaving Israel. However, the word "Galus" is also used to refer to any forced-departure from one's domicile, even to go to Israel (when there is no Beis ha'Mikdash). Being forced to leave one's home in Bavel (as Rav Kahana was, see Bava Kama 117a) to go to Israel, therefore, is also appropriately called "Galus." (This is the context in which the word is used throughout Maseches Makos, where the punishment of Galus involves running away from home to another city inside of Israel.)
The fact that he was forced to go to Israel, and thus calls going there "Galus," does not diminish the fact that it is also an "Aliyah," since one fulfills a Mitzvah by living in Israel, and "the air of the land of Israel makes one wise."