If Poe's not 2nd in charge, you have to explain how he's in charge after Leia.
As far as compartmentalizing the plan from your leaders, this movie explains exactly why that doesn't work. Even if Poe was the mole, or regardless of who the mole is (if they had one), as soon as they start boarding the landers, the mole would let the FO know what they were doing. If they had a mole, the FO would have known the plan before DJ had a chance to spill it. So hiding the plan from Poe would not have helped them regardless. By shielding the plan from key leaders, you remove any chance of helpful input they may have to help improve the plan, and increase the risk they'll do something to spoil the plan, because they have no idea of what the plan is...which is what happened.
Holdo = 1 bad decision after another, ad infinitum.