1) Indeed, if you have one midrash, you don't need the other. This is likely a disagreement, rather than an assertion that both happened.
Rabbi Yossi ben Hanina, the one who stated that it was tzaraat, is an Amora of Eretz Yisrael. As Tosafot notes on the daf, in the parallel Yerushalmi, we find out that this is a derasha on the word nigzar in Esther 2:1:
וְאֵת אֲשֶׁר-נִגְזַר עָלֶיהָ
The word nigzar also occurs by Uzziah, a king who brought ketores, though he was not a kohen, and was stricken with leprosy.
Meanwhile, the brayta presumably derives the tail from some other source. According to Tosafot HaRosh, it is the word עָלֶיהָ in the same phrase. Written with an aleph instead of an ayin, it would be read alya, which means a tail.
2) A person grows a tail suddenly as a result of a miracle. While I'm sure some people explain the significance of specifically a tail, I would personally note that the rules of midrashic interpretation will restrict the details of the midrash to specifically that which can be deduced from the pasuk via midrashic rules. This was a means of suddenly marring her beauty, such that (based on the context of the gemara) though she would normally have loved to lewdly display her naked body, here she was embarrassed.
3) It depends. I would say yes, the rabbis who wrote this did intend them literally, though we do not have to agree that it historically occurred. Alternatively, it is meant metaphorically. See my discussions of this here and here.
4) I don't know. Different midrashic authors might mention different details. I don't know what specifically brings Gavriel in here, or if there is a running tradition of Gavriel secretly guiding all sorts of events in the megillah. For example, see Megillah 16a, where Shamshai the scribe erased what Mordechai had done to save the king from Bigtan and Teresh, but the angel Gavriel came and rewrote it. Note also that the entire second answer is in square brackets in our gemaras. I am not sure what that means, or what alternate girsa exists there.