Tim:
this idea walked through my brain during so few time that I forot it. I may use it after all (since I was not able to report the found solution [from memory] earlier today).
It seems to me that I was able to implement in the real project the code I create in a new project created to experiment the idea.
In App.Open, I set the Boolean to the appropriate value (True I think, the property name is NoAuth).
In the OK PushButton, I check if the Password typed by the user is correct. If so, I set a Boolean (an App Property) and act appropriately (in a If Else block).
In App.EnableMenuItems, I Enable / Disable ALL MenuItems in a If Else block.
If done correctly (respect the Boolean: True and False), it works fine: the menus keyboard shortcuts are no more fired from the Password window, and are working fine once it was closed.
Thank you all.