marcman wrote:For example, if Yvraine is Warlord in a harlequin detachment, can she use player of the twilight Warlord trait?
Yvraine can't use any of the warlord traits from the codexes, because Yvraine doesn't meet the definition required to be able to take one.
The latest errata allows for the detachment itself to remain a Harlequins/Craftworld/Drukhari, but Warlord traits care about the model, not the detachment.
Page 116 Codex: Craftworlds, Page 114 Codex: Drukhari and Page 68 Codex: Harlequins define what a Craftworlds/Drukhari/Harlequins unit is.
Page 121 Codex: Craftworlds and Page 124 Codex: Drukhari state that only a Craftworlds/Drukhari CHARACTER may take a Warlord Trait from the list.
Page 74 Codex: Harlequins is even more specific and requires that the model be a HARLEQUINS CHARACTER.