So I've heard many times that the Talmud is sealed and there's no arguing with it or repealing rabbinical decrees. But I've also heard that when Moshiach comes his court will repeal many decrees. But how does he do this if it's sealed? Even if you say when the rule was made it was intended that Moshiach be an exception to the rule, but I believe I've heard in the beginning we won't know he's Moshiach. Does that mean he only begins repealing decrees after absolutely proving he's Moshiach?
A future court may overturn a ruling of a previous Sanhedrin, if it is greater in wisdom and number than the previous court. (Talmud Tractate Ediyus 1:5)
Moshiach will eventually reach that qualification.
Since we do not consider ourselves greater in wisdom and number compared to the Sages of the Talmud, we view it as "sealed" for all intents and purposes.
When I started my Talmud study I also thought so, but the Talmud is not what you think. Talmud is sealed the same way any other book, like [להבדיל] "The Old Man and the Sea" is sealed. The text is fixed.
But what you do with the text and what Halachot you derive from it is open for everyone. This is the reason the Rishonim (Ri"F, Ro"sh, Rambam, Rahi etc) learned Halachot differently from one another on the basis of the same Talmud.
When Moshiach comes he can offer his understanding of the Talmud that will form new Halachot without altering the text of the Talmud.
To your premises:
"there's no arguing" - there's no need arguing, the Talmud is extremely vague - everyone can understand the same statements differently and arrive to contradicting Halachot.
"repealing rabbinical decrees" - no need to repeal. You can always say [the same way Rabbis treat the Mishnah] that a decree only refers to a private case, and rule the Halachah otherwise. BTW the number of clear and final rulings in the Talmud is very very small. We don't rely nowadays on the Talmud alone but only on the Rishonim and the Achronim.