I’ve not seen “II” in a while, so I popped it in the DVD and watched the relevant scenes.
Michael as much says to Tom that he’s sure it’s an inside job, as you describe, so your scenario makes sense from that standpoint.
Tactically, and strategically:
Flipping part of the Corleone’s Household Security shouldn’t be that easy; Michael is smart enough to buy loyalty, and ensure that his people (the ones who would be close enough to kill him, or his family, at any moment) are the type that stay bought.
Given that, any member of the Household Security Force who wasn’t where they were supposed to be at the time of the shooting would come under immediate suspicion. That would include whatever “security guard” may have clipped the two “shooters.”
Judging from the movie, anywhere from 15 to 30 seconds pass between the shooting and the front gates being closed, and the compound lights being turned on. The property itself is lakefront property. The dogs are kept in a kennel; they were only released after the shooting. There’s no on-screen evidence that the night patrol had dogs with them (a serious oversight, IMO).
A “commando” type group could’ve infiltrated the compound and done the deed. Say, three men in a black rubber inflateable, with an electric outboard; the compound lights are out, and it’s a rainy night. Two shooters go in, one man holds the boat. Unbeknowst to the “shooters,” the “boatman” has orders (say, from Johnny Ola) to kill the two “shooters” when they return to the boat. He does so, gets in his boat, and putt-putts off into the night before the Security Force can get the motorboat out and start patrolling the shoreline (which we see happen in the movie).
The crew on the motorboat are also seen to be pointing the searchlight in towards the shore, not out into the lake. Our hypothetical commando “boatman” could be well on his way with the cover of night, dark clothing, and rain to keep him covered.
Strategically, it also makes a kind of sense to do it that way.
Let Michael (or his minions) chase shadows and nonexistent traitors in their own ranks while you quietly dispose of the “boatman” as well. Only you, as Hyman Roth, and Johnny Ola, ever know that anything took place “from your end.”