events and workflow are not supposed to be in the 5.1.2 Core tarball, because they aren't core.
"is there a particular tag I should be using to get something stable"
Yes, "oacs-5-1-compat". This is also the tag used to rebuild the repository, when that is working.
When a new version of OpenACS is released, we should also tag whatever is openacs-x-y-compat at that instant with the -final tag, so that we permanently identify the stable code at that instant. I left this out of my release docs and so skipped it when I released OpenACS 5.1.2, but I've added it now and done the tagging, so openacs-5-1-2-final should now extend to all packages.
