So if you kill a marked target you have a 25% or 50% chance to immediately strike again. Does anyone know how this interacts with multi-attack champions like Jarlaxl, Binwin, and Drizzt (assuming you select the multi-attack option)?

I would almost assume that it sets your attack timer to the top when you make the kill but for Binwin you could proc more attacks delaying for 2 seconds per additional attack. Jarlaxl would attack again anyway though the setting of the timer to the top would work as expected. As a worst case scenario, if it interrupted the current string of attacks, you would miss out on attacks.

I've been trying to watch and I don't think it interrupts Jarlaxl at least but I haven't been able to tell for the others.