It would depend on whether Tom Hagen and Neri would be able to hold things together. I am ssuming Fredo was in on the hit (as I have for some time now) and Fredo being the bonehead he is, would have tried to move in too quickly to "take over," and if Hagen and/or Neri were astute enought they would see Roth was behind.

Tom probably had an idea about what Michael was doing in Cuba, and if he could control Neri and convince him it was Roth, then Hagen, Neri and Rocco could use Pentangeli in NY to do what Pentangeli wanted "hit 'em while we still have the muscle."

It would be an uphill battle though, because with Michael dead the Corleones could not move in on Klingman because Geary was in the way. If things fell apart, I'd guess Roth would kill Rocco, Neri, Hagen, and eventually Fredo and take it all over.