Jim, bet you won't get anyone to touch this one. No one will want to
be responsible if your mirror DOESN'T hold and they have given you bad
advice. I cannot imagine you telling your SO that you explicity
followed the instructions given to you by someone in Chatanooga for
hanging your mirror.
If there is any doubt, by the etching kit. It does two things: cleans
the metal to remove all traces of hand oil, manufacturing oils, dust,
dirt, etc. It also literally etches the metal to provide a grippable
surface for the adhesive.
I know when we put in a shower/tub enclosure and the aluminum is not
properly wiped down we even have problems with out gooey sealants
Why gamble with your project? Maybe you can recess the hanger about
1/8", and then adhere your channel. I think I would feel better about
that then just glueing it on. Of course, I would etch, too. Maybe
you could find the etching material at one of the metal shops in your
area if you feel West's is too high.