Is EPEL "upstream" or "officially package repository" (like Fedora Extras was)?
EPEL is just one of several add on repositories with RPM packages for RHEL and derivates is not a official repository. The different repositories serve different users bases or follow different ideas.
Just like RHEL itself, EPEL in reality is more a "downstream" in the sense that Fedora is upstream and EPEL just like Red Hat takes packages for its product that are constantly developed, tested and received feedback in Fedora. Red Hat through their sponsorship for the Fedora project and participation of Red Hat maintainers continues to back EPEL, but Red Hat has not endorsed EPEL or commercially supported it.
The EPEL maintainers are further well aware that EPEL can't serve all needs and that other repositories are likely needed for kinds of software the Fedora project won't provide which includes packages for EPEL will a rolling release model or non-free and patent encumbered software.
Why doesn't EPEL use repotags?
There were a lot of long discussions in the months of EPEL about using repotags or not. Lots of people from inside and outside of Fedora and EPEL as well as maintainers from other repositories participated in those discussions. No real agreement could be found if the benefits outweigh the disadvantages -- part of the problem was that people sometimes not even could agree what benefits or a disadvantages repotags have (or if there are any). The final decision in three voting's (one done by FESCo before EPEL had a Steering Committee and twice done by EPEL's first Steering Committee) was to go without repotags in EPEL.
How can I find out if a package is from EPEL?
If you want to find out if a package comes from EPEL package use a queries like this:
$ rpm -qp foo-0.1-5.el5.i386.rpm --qf '%{NAME}-%{VERSION}-%{RELEASE} %{VENDOR}\n'
foo-0.1-5.el5 Fedora Project
$ rpm -qp foo-0.1-5.el5.i386.rpm --qf '%{NAME}-%{VERSION}-%{RELEASE} %{DISTRIBUTION}\n'
foo-0.1-5.el5 Extras Packages for Enterprise Linux
$ rpm -qp foo-0.1-5.el5.i386.rpm --qf '%{NAME}-%{VERSION}-%{RELEASE} %{SIGGPG}\n'
foo-0.1-5.el5 883f030500468e3e4e119cc036217521f611025863009f5fe424c6fe4bc81a57f45722e465e71381dda2f6009f7c08e1743794b5b9a5a4cd149081092801a5d935
Is EPEL willing to cooperate with other third party repositories?
EPEL is always willing to discuss cooperations with other parties and repositories and encourages maintainers to do so whenever possible.