The impression I got from the Great Harlequin's opinion of Motley was [spoiler]the Solitaire protects the Harlequins from Slaanesh by basically being Slaanesh. In effect, they play the role, giving a concrete focus for what is Slaanesh and thus something for the other players in the troupe to basically "not be". In effect, "I can't do that because it's not my role", this the Quins are all "exarchs" if you think about it from a Craftworlder perspective, they're literally defined by their roles and protected by being so defined.
Solitaires are Slaanesh magnets and by playing that role they protect the rest of the troupe by being the one Slaanesh goes for first, they suck it all up, they hide it under their "masks" (a la War Masks from the Eldar Path series) and eventually they break. [/spoiler]
As for the last chapter [spoiler]it's brilliantly told.
You the read end up as part of the Masque as much as Vyle, you don't know whether it's a performance of if Motley actually has flipped out and gone fully Slaanesh.
We're told repeatedly that the non-Harlequin players end up sucked into the performance, and then instead of watching it happen, it happens to the reader.
Alternatively, Motley's just a little piece of Slaanesh now, and everything he did in this story and potentially the DE Path series was Slaanesh seeing if they're as good as Tzeentch in manipulation.
After all, in Path of the Warrior, he breaks the Spirit Mind of a Exodite world.
Then in Path of the Incubus he repairs it through an injection of Chaos corrupted souls (Morr and the Eldar Warlock).
Incidentally, that first action? Causes a Disjunction allowing lord only knows how many daemons into Commoragh, and fixing it closes the Disjunction.
The only thing going in the favour of Motley not being fully corrupted in the DE Path series is that he manipulates Morr into slaughtering a DE Lord whose been corrupted himself.[/spoiler]