For an emacs user, that's pretty exciting.
Replacing Emacs elisp implementation is just looking for trouble. Although it is not the fastest one, the existing one works well enough and is battle tested and there are millions of lines of elisp code that would need to be tested, debugged and fixed.
If the goal is to allow to write parts of Emacs and extensions in some other language, I wander is Scheme really the best choice? Why not go all out and implement elisp in Common Lisp and use Common Lisp as a base for future Emacs and extensions development. There have already been such attempts, so it's not exactly something unheard of.
Emacs' sheer size (which you mentioned yourself) means you're not going to port it to a different language and have something comparably useful in less than a few years, assuming sustained interest and full-time development.
Guile's REPL is among the best, I think. It's all documented here:
But for real interactive development, you want Geiser:
You're going to be excited for an extended period of time. I hope you've got a strong heart, for your sake.
(I guess it would, as much as Mono can load PE .NET files for OSX/Linux, after all these won't be loaded directly (it won't make much sense without the runtime anyway))
ELF <-> PE is doable.
any debug format -> PE|ELF is doable (linking + stripping)
com|a.out -> PE|ELF is doable