First of all, this sounds awesome, thanks for doing this!
We have a library interface to the major parts of SwiftPM, called libSwiftPM, exposed as a package here. You could also use the "./Utilities/bootstrap" script in the SwiftPM repository to build the dylib and integrate that way. If you are using the second approach, you'll also have to include the llbuild framework.
With this, you should be able to accomplish your second objective, but I don't think SwiftPM's dependency resolver is necessarily set up to be used by a third party package manager right now.