The alkaline ammonium fix with hardener is not practical. You cannot use alums on the alkaline side, and aldehydes tend to polymerize on the alkaline side in the presence of ammonium ion. Therefore, I know of no good alkaline hardening fix based on ammonium thiosufate.

In regard to ammonium 'bleaching' silver faster than sodium, IDK. It depends mainly on pH AFAIK and activity, and so I would think that if bleaching were to take place it would be faster in acid and with ammonia present. I have not tested it personally. Textbooks show this but don't make all of the necessary comparisons to be able to properly define the rate of any fix in bleaching silver. I know that it is rather rapid! More so than most of us would expect.