I have tapped legend LeBlanc and then conquer Hall of Legend battlefield, can I trigger Hall of Legend first to ready my legend and then use her ability?

No — if LeBlanc is already exhausted when you conquer Hall of Legends, her Deceiver trigger never enters the Chain at all.

"Exhaust me" is a cost paid to finalize the trigger (Rule 383.3.b), and an already-exhausted Legend can't be exhausted again to pay a cost (Rule 414.1.b, 414.4). So Hall of Legends' "ready your legend" trigger is the only one that fires — it readies her after the fact, but the conquer trigger window for Deceiver has already passed. This is the same interaction as Rek'Sai (FAQ #11259).

If LeBlanc is ready when you conquer, both triggers fire simultaneously and you can order them freely — e.g., Hall of Legends resolves first, then Deceiver exhausts her and plays the Reflection.