I think you downloaded the creepypasta version of Miku.
On a more serious note, I did some experimenting on the model and I suppose I came to a conclusion. Firstly, you can fix the shoulders with welds and smoothing fixes. The model is scattered across the first 16 or so smoothing groups and you really only need 2-4, more or less depending on the model’s complexity. In other words, just set everything to 1 and go back and modify smoothing groups on necessary areas to prevent nasty dark spots. The skirt and clothes are basically just a plane, so you don’t need to worry about those areas. You can use $nocull one in on those materials to draw it on both sides to fix that in-game.
As for the face, it’s just built extremely awkwardly. The “face” is one static object. The mouth isn’t a dynamic part of the face. Instead, the lips are welded together and the real mouth sits inside her face under a probably transparent layer of lips. The eyes are also completely separate from the face to make it like she’s wearing a mask for absolutely no reason. This might make things extremely awkward in-game, but you might be fine when it’s in-game if you can get lightwarps working correctly. You’re probably going to have to use an awkward method of toon shading though to make sure her face is lit consistently to avoid the strange shading you see in 3ds max.
As for the eyebrows, you probably don’t really need to weld them to the face. The topology there isn’t going to be happy without reconstructing it to compensate for the eyebrows and there isn’t a fully painted material for them. If you don’t plan on using face flexes, you could probably just move them closer to the face or even leave it as is – it’s not bad enough to cause any notice, really.
Honestly, before I rig a model, I usually compile it as a static object and start playing around with the materials to make sure I’m happy with how it looks in the engine so I know what edits I might need. I’d give that a go to see if you can get it look how you like. You may not need to worry about fixing everything as much as you think (unless of course you’d rather rebuild the face anyway).