Well, I am just in front of Manimarco's throne room.
Several times come to the same end: when I ki8ll him or he notices me, a paralyse (probably) spell is cast, I am knocked down. When I got up, I cannot move.
Do I just wait for the effect to wear off or... what?

Edit: even if i kill everybody in the throne room including Manimarco, when I start climbing the stairs to the throne that nasty spell hits me, I fall, get up but cannot move, cannotopen menu, cannot cast spell...
what next?

After that spell, Mannimarco is supposed to engage in dialogue. If he isn't, try entering enableplayercontrols and manually talking to Mannimarco.

Phew! Even this did not work. Then I made a save and started enter/exit until finally Mr. Manim. finally started talking to me!

I suspect thatsomething in thescript isdue to be fixed.
Otherwise - vicious fight even with my superhero of char. Good fight.