I've implemented your advices and modifications to the core of user merge feature and now it's working on oacs-5-2. This core includes modifications in acs-tcl, acs-kernel, acs-authentication and the GUI (acs-admin). It is commited and tested for pg and oracle.
I've tested the sql install and upgrade scripts too (oracle and pg). They are 'drive&run' ;).
In Addition of the core of this stuff, I'll commit, as soon as possible, the 'MergePackageUser' and 'MergeShowUserInfo' implementations of most of the packages.