Just take a checkout via:
cvs co -r oacs-5-1 openacs-4
and submit bugs as found in "5.1".
I think you are a little confused on terminology: you
can't commit on "5-1-compat" it's not a branch, the branches
we currently are working with are oacs-5-1 and HEAD.
openacs-5-1-compat is a "symbolic tag" and serves as an alias
for the latest versions of a package which have been marked
as compatible with the 5.1 release. You can only commit to
a branch -- someone then needs to update the opeancs-5-1-compat symbolic tag to point at the latest version
of that file on the branch on which the 5.1 compatible version of that file lives (for everything except project manager that
symbolic tag should be pointed at the latest tested version on the oacs-5-1 branch).
In terms of testing uninstall, yes, I think the uninstall should work even if it's mounted so don't unmount things
first -- just do the delete via the "Uninstall this package"
at the bottom of the version view linked from the /acs-admin/apm/ page.