So if the warden's LI is Leliana she isn't the Left Hand of the Divine? Or when DAI starts do they set apart, the warden going to the westen lands and Leliana becoming the Left Hand?
I'm asking because I'm really curious and I was planning to make Leliana my LI until I start to hang around with Morrigan and Alistair (oh, those banters!). So when I had the chance I broke up with Leliana and went for Morrigan 
I haven't finished the game yet and I know I should play it again and see by myself but job+RL problems = less and less time for gaming 
If Leliana and the Warden are together she's still the Left Hand, and has been for some time. In Awakening the Warden will get a letter from Leliana saying she's been summoned by a Grand Cleric; I assume that's Justinia before she became Divine, and that that is when Leliana was recruited as Justinia's agent. She's almost certainly Left Hand by 9:37, which is when Justinia sends her to Kirkwall to assess the situation.
Tallis and Leliana will recognise each other at Chateau Haine in Mark of the Assassin. Since that can happen as early as 9:31 I assume that Leliana was operating as Justinia's bard since the end of the Blight.
if Leliana and the Warden are together, when the Inquisitior asks about the Warden Leliana will say she plans on joining them when her duty is done, because she's lost too much and refuses to lose them too.