1) I think the WTFPL licence is pretty clear about whether you can use files licensed under it in any projects - you can do whatever the f... you want to, so yes, you can :)
2) Another story is that, if I understand correctly, it is you who wrote both the WTFPL code and the MIT code. In this case, you can do whatever you want with both parts of the code, since you're the author and you keep the copyright to the code. In general, if you release any project as MIT/BSD/GPL/WTFPL/whatever, it doesn't mean you can't include that code in a completely closed source application without asking anyone for permission - YOU are the author, so you only need to ask yourself. This changes once other people start contributing to that project - but as long as 100% of the code was written by you, you're free to do what you want with it. Releasing under an open source license doesn't take that away from you. (Of course, if you release a project as GPL, and then make a paid closed source project out of it, it doesn't make much sense - because people will just use the GPL version - but there's nothing illegal about it).
3) It's not clear to me what's the status of code licensed under A included in a project licensed under B - if it's licensed under both licenses, or what... but I always keep the original license header in this case, i.e. in your case I'd keep the WTFPL header in the file when it's included in the MIT project.