I'm at the final resting point and having a hard time collecting mystery goo. Even when I get the mushrooms it's tough to get them to give me anything, so I'm wondering if it's worth it to complete the final battle and Hades Cup.
Um, it is worthwhile to complete the final battle since you've come all this way. You should do the Hades Cup if you want the secret ending. As for everything else, including the ultimate weapons, that's entirely up to you. They do make things easier, but you can do without them.