( I am 100% sure I chose the wrong component, but I don't see anything more appropriate, I am very sorry for this ).

menu is shown on xMB press not on release

The action like showing context menu should be executed on release, not on
press. 
a) it is about consistency -- all elements in environment like KDE are activated on release (links in web browser, window decoration) with only one exception -- menus

b) is not about accessibility -- when there is immediate reaction, there is hard to work in user pace, it is also harder to make emulation of MMB happen (even with some timeouts set)

ad.b) I didn't realize how hard to is to press both buttons (LMB+RMB) in KDE (Konq. actually to open page in new tab), because I had excellent mouse -- with literally three buttons (old unix school). Now I have two buttons mouse and pressing both of them hurts my hand -- it would be much easier to be able to press&hold RMB and then press LMB and then release them both.

So fixing this issue would have impact both on consistency of UI and accessibility. The first is rather a bug (flaw really), the second a wish.

I posted this report at KDE bugzilla, with answer that it is up to Qt. Qt team answered they are willing to provide configuration option, how menu should behave and this is valid report, but they will wait for you, for providing standard way to do this.

Just in case I would not be understood:
"
you should address this issue to the Freedesktop project
(http://www.freedesktop.org). If freedesktop defines a standard way to
configure this, the changes in Qt are trivial.
"

So I hope I finally reached the right place to ask for the change ;-)