This problem seems to have been solved:
"Just to update on the OpenIHM issues, I've found a workaround which
involves taking ownership of the Python folder and granting 'Full access' to
the local 'User' account. It seems the Python folder is being installed
with no access rights for users."
The problem with mercurial-2.3.1.win32-py2.7.exe was a 'BEX' error, with the
installer trying to make changes to something it didn't have permission to
access.
Best wishes,
Wolf