Apple, with all due respect, it WAS Michael's plan to have Roth met at the plane. However you are correct in stating that Michael didn't care how it was done...as long as it was done.
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Here is that part of the transcript :

TOM : That plane goes to Miami.


MICHAEL : That's where I want it met.


TOM : Mike that's impossible -- they'll turn him over to the Internal Revenue, customs, and half the FBI.


MICHAEL : It's not impossible. Nothing's impossible.


TOM : I'd be like trying to kill the president -- there's no way we can get to him.


MICHAEL : TOM, you know you surprise me -- if anything in this life is certain -- if history has taught us anything -- it's that you can kill anybody. ROCCO?


ROCCO : Difficult -- not impossible.

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So obviously Micheal wanted it done there, when Roth got off his plane. And in saying " ROCCO?" Michael is asking Rocco if it can be done. So the actual plan of HOW Roth would be killed was Rocco's plan, but WHEN Roth would be killed was Michael's order. I am sure that Michael was aware of how Rocco intended to take out Roth. Michael was very cunning and calculative, and had to realize that Rocco's plan would in all likelyhood turn out to be a suicide mission. So why would he allow one of his two TOP men to sacrifice his own life in that way? had Michael really become so cold and uncaring that he wanted Roth dead even at the cost of one of his right hand man's life? Or was Michael allowing Rocco " a way out " just as he did with Pentangelli!?!

Don Cardi