3 deliberative desires, 1 ability to will?

I have been reading Andrew Moody with interest. Here I parade my ignorance about a couple of points!

Moody seems to want to think of the persons of the Trinity as three willing agents (who share the one (natural) divine will?), or of three minds, so as to avoid Modalism (that is, so as not to collapse the persons into three masks or appearances of the one God). (As with wills, I believe it would be traditional to think of one divine intellect, though of course existing in three persons).

Moody cites Maximus the Confessor's distinction between deliberative desire and the ability to will.

"Deliberative desire," he [Maximus] says, belongs to the hypostasis as the ability to will belongs to the essence; Opuscule 3, PG 91.48A-B. Thomas Aquinas makes the same distinction using different terms in Summa Theologiae 3.20.1. [Footnote 4 of the blogpost]

Now that, I need explaining to me, please! What is the difference between deliberative desire and the ability to will? And why would we want to say there are three deliberative desires grounded in persons and one ability to will in the divine essence? Is it just a way of saying that the Son could not disagree with the Father? If so, fair enough, but I'm not sure it adds much.

I'm inclined to think that one could imagine three modes or manners of having or personalisations (or something like that) of the one divine will. I'm not sure what the best terminology would be. Nor how exactly this differs from what Moody is suggesting here.

(I am also inclined to the idea that God's will is entirely necessary, which Moody rejects in preference for a particular version of making God's will to create "free". Moody is with the majority here, I believe. But it does not make sense to me. If God's will is perfectly at one with his wisdom and goodness it seems to me that God freely and necessarily chose to make the best of all possible worlds. God could not have done otherwise, but that does not mean he is compelled from outside. He does what he pleases in conformity to his omnipotent wisdom and goodness. But that is probably a distraction from this present discussion!).

I could also do with a lot more thinking about what the Fathers meant by "persons" with respect to God and what we should mean by it. I wonder how helpful "acting agent" is, but I can see the danger of making "persons" an (almost) empty category. I think we feel a strong desire for the persons to feel personal - even almost like personalities - but I have the sense that this was not quite what the Fathers meant? Obviously all the good guys in this debate want to avoid both the Modalism mentioned above a Tritheism which 3 agreeing people would suggest.