Can Rumble Hothead's conquer trigger be activated when there are no mechs in the graveyard, with the intention of ordering Zaun Warrens' trigger first to discard a mech from hand, then targeting that mech with Rumble's effect on resolution?

Ruling: No, this does not work. Rumble Hothead requires a valid mech target in the graveyard when the trigger is declared and added to the chain, even though the effect itself is optional ("you may") on resolution.

Sequence:

• When conquering with multiple trigger effects, you choose the order to place them on the chain
• Each trigger must have valid targets when declared/added to the chain
• The chain then resolves in reverse order (chain 2, then chain 1)
• Optional "you may" effects decide whether to execute on resolution, but targeting happens at declaration

Nuances:

• This differs from the Monastery + Sett ruling because Rumble specifically targets a unit in the graveyard at declaration time
• You cannot declare Rumble's trigger without a valid mech target already in the graveyard
• The "you may" on Rumble only applies to whether you execute the effect on resolution, not whether you need to target on declaration