Axis & Allies Developer and Playtester; War of the Ring Editor and Playtester
1)Can the Shadow player to employ Dunland in the besieged citadels of the Free Peoples player (in Isengard and Helm's Deep)?
If a Shadow Army is besieging a Stronghold in Isengard or Rohan, Dunlendings can be mustered in the besieging Army.
2) Can the Free Peoples player trebuchets to employ them in the captured citadel Shadows?
No. They can only be placed in (originally) Free Peoples Strongholds.