Once per turn: You can Tribute 1 other "Chronomaly" monster, then target 1 face-up monster your opponent controls; its ATK becomes 0 until the End Phase.
Player A has on field Chronomaly Cabrera Trebuchet, and 1 more Chronomaly monster, so total of 2 monsters on his field.
Player B activated Enemy Controller and took control of Chronomaly Cabrera Trebuchet.
Q1: It says
You can Tribute 1 other "Chronomaly" monster,.. without specifying which side of the field, so player B can tribute player A's other Chronomaly monster to fulfill this cost, right?
Q2: For any card in YGO, can you fulfill a cost without doing the effect? Like in this case, player B wants to fulfill the cost of Chronomaly Cabrera Trebuchet by tributing player A's monster, but player A doesn't have any more monsters to change their attack to zero. Can player B still activate Chronomaly Cabrera Trebuchet's effect?