Ok, this part of the movie to me always seemed to leave tremendous pieces of the puzzle for the viewer to figure out on his/her own.

My conjecture is the following (and I would appreciate criticism on this):

First of all, I presume that the two would-be assassins found in the ditch were going to be murdered anyway. They had no way of telling whether or not they had killed Michael Corleone since they sprayed the bedroom with bullets. Thus, regardless of the outcome for Michael, they were going to end up in the ditch with their throats cut.

If it is safe to assume that Roth was indeed behind the hit, as Michael presumed he was, I would assume that Roth wanted the gunmen left as "bait" for whoever might be directing the family affairs after the shooting. Since the gunmen appeared to be "hired out of New York", this would present the appearance of Pentangeli's involvement to the Corleone family, misdirecting possible vengeance from the Corleone's.

Thus, if Michael lived, he might strike out at Pantangeli, or even let the Rosato brothers do it for him, which would be exactly what Roth wanted. Also, Roth probably figured the "new york" gunmen could misdirect Michael long enough to cause Pentangeli's death before Fredo squealed about his conversation with Ola, should Fredo indeed grow a heart and do so.

The other eventuality - Michael's death - would truly be a strange predicament. Who would assume control: Hagen? Fredo? Since Hagen was 'out' of the dealings with Roth (by Michael's orders) Hagen would be in no position to assume it was Roth behind the hit. Hagen did, however, see Pentangeli acting drunk and disrespectful at dinner, and heard Pantangeli give Michael an earful of insults during their business meeting. The "new york" men would probably be a link causing Hagen to go after Pantengeli. Again, Roth would benefit. If Fredo were somehow to have control, perhaps Roth could fool him into thinking it was not actually him behind the hit that night.

Lastly, I believe the assasin's assasins escaped by boat. The complex seemed to be relatively secure along all its land borders, but had a large border along the water which would make a convenient point of invasion. It would take a while to fire up the compound's boats and chase after anyone in the water. Also, due to the time period, the lighting equipment on the boats would make such a chase near impossible at night.

In the end, it seems like Roth put a lot of thought into striking at Michael.