I have something to say to you.
First, I was really shocked when you said 2 stages in 5 years then I went to check out the videos you have for the game. After seeing the stages I was, oh now I see why it toke him this long (why would you even consider making such a huge game if you don't even have the resources to pull it off?).
Anyways going back to my analysis.
Stages are way too big and complex (I'm not going into more detail because that is not the point of this post). Each one of those could be broken down to 2 different levels. For example, why have 2 completely different stages for Boltman, You could easily make the Tower his stage and use the valley for another master. Same goes to Duplexman stage, can have the ride be one stage and the other half another one. I'm not trying to break up you game, destroy all your hard work or anything, I'm just suggesting possible solutions to have a more complete game because at this rate you will never finish.
Also, you need to plan out stuff in a broader range and be able to adapt. Doing one thing at a time might seem effective, but not when you're working with others and depend on them. People tend to get bored when doing the same thing over and over. This way you can have multiple people doing different things. Also, not everyone knows how to sprite a graveyard for example but maybe a cave or a forest for another level, but since you're stuck on that one stage everyone else is stuck also.
Another big "mistake" us amateur developers do is go out of scope and do fun things that are not originally planned. I read you made a time attack and now wanting to do this "walk around lab thing" which all sounds cool and amazing but push the release date even further behind.
I have a lot more to say that I learned from years of experience doing amateur games (MMXC was not my first project) but that is pretty much what I wanted to share, take it as an advice, criticism or whatever.
Sorry for jumping around, hope I made at least a little bit of sense.
Best of luck.