from recall, (at work) most units with banners or icons say
a figure may carry either "x" or "y"
so I'd think a single figure cannot carry both
but you can have both in a unit , as they are on 2 different figures (I do this with my Slaanesh Warband)
as to champion/leader/prime,
(I have my Stormcast Battletome with me)
it looks like the champion CAN have one of the special weapons,
as well as its champion bonus (+1 to hit or +1 attack)
I don't have the Tzeench book,
but if the Twistbray is the champion equivalent,
and the warscroll states that "x models can be mutants"
and "y models can have (special weapon)"
*I* would say you can have a Mutant Twistbray with a Savage Greatblade
opinions may vary?