Simple; Duvalier is a mortal incarnation of Yahwah, who doesn't like how the cult is using his holy book. Or Duvalier is Anu, who doesn't like the way the cult is acting. Like Jesus, but in telepath. So he knows everything and has unlimited power, but restrains himself.
Of course, the above is probably nonsense (Though Craig will never tell us so).
Anyway, it seems clear to me that the thing that Duvalier actually does is mould the unformed energy into a roughly compatible form so it takes with the body, and reanimates it, also rejuvenating it. But that is unimortant, as Luca still does something Duvalier can't. Now, back to the main debate:
In any case, the problem with your theory is that it does not take into account that the spirits shouldn't need bodies at all; if it can exist independently of the body, which it must to be able to use psy to create a body, why does it make itself vulnerable by creating a body at all? Habit? No, as that kind of power would have to be focused - creating a body out of a mind shield and energy would take power and concentration.
I don't understand why assume that spirits don't need a body. Clearly, when a spirit's body is destroyed they can't do anything anymore. Luca can use multiple skills untill her body is destroyed. Untill Luca can gather all her psy energy back into a human form, she is useless.
Please don't use the word 'clearly' without presenting solid evidence. I'll argue with the scrap you did present in a moment. There wasn't nearly enough evidence to use 'clearly', and the assuredness of your post comes across somewhat irritatingly when there is nothing whatsoever to be sure of. You didn't even counter my argument, you just posted a fact which supported your view. Clearly.
Luca cannot act because all her power has been consumed sustaining her body - if we take your point, that it is a shield, to be fact.