Detecting the proper mode of operation
As mentioned in the proposed solution page, there are reasons that the canonical source should be mutually exclusive between either an srpm style source or an scm repo style source.  I therefore propose that we key off of the presence of the various SCM related tags in the spec file (or the rpmdb) to indicate to both rpm -i and rpmbuild whether to treat a particular package as an srpm style package or scm package.  This eliminates any ambiguity in the build process for rpmbuild in particular.
For rpm -i the mode of operation merely selects the method of installation and sets a few defaults in terms of installation paths.
Likewise for rpmbuild, the primary difference for the different modes are the defaults paths to things like sourcedir, builddir, and specdir.