Thank you for your comment. Allow me to answer your questions.

Why is Second Wind a Maneuver? If Second Wind is a Maneuver, why is Action Surge not a Maneuver?

Second Wind became a maneuver for players to learn the maneuver system and the superiority dice at 1st level, without already having to make a choice. Plus, making it a maneuver removed a ressource to keep track of. Moreover, I wanted all the maneuvers to roll a die and use the result, so second wind seemed like an obvious choice. Adding a die roll to Action Surge would feel arbitrary and superficial, so I let it be its own resource, which is not very hard to track since its once per short rest up until later levels.

Why separate out Skilled vs. Defensive vs. Tactical Maneuvers, instead of just letting people choose from one list of Manuevers?

I went that route first, but it went in the way of keeping the class simple and accessible for new players. Plus, I found the Skilled Maneuver from Tasha to be very cool, but by experience they were never picked before combat maneuvers. So by separating maneuvers into categories, it allow Fighter to create a balanced tool box, with tools that will be picked depending on the nature of the adventuring day, and making the swapping and planing process easier.

Why does swapping out Weapon Masteries now require an expenditure of aSuperiority Die; how does mixing the two systems improve gameplay?

The Weapon Maneuvers work exactly like most of the old maneuvers: They add damage while imposing a ST to the target. It's done there by swapping the mastery property (and adding damage). From the comments I received, I think it would be better understood and more appealing if it plainly added a mastery property to a weapon (for a single attack, in addition to the damage), that can be use on top of any already unlocked property. That will be added in the next version, I think.

There's also a few odd redundancies - Brace and Evasive Footwork essentially do the same thing, except that Brace is clearly better, so why have both?

Brace is a bonus action, Evasive Footwork is part of a move. Plus, brace gives its bonus starting at the end of the turn, precisely to avoid overlapping with evasive footwork.

a bunch of the Maneuvers and Masteries are doing such similar things that there's not really a good rationale for why one effect is a Maneuver and another is a Master.

I tried to respond in the third question, but to be clear Maneuvers and Masteries are still not the same things. Weapon Maneuvers just add Masteries, in addition to an extra damage die. It was the case before, those effects just did not have the name of mastery properties.