“If Roth had succeeded in having Michael killed--either in Tahoe or Havana” Roth would have done us a huge favour! – No Godfather 3!!

My take, for what it is worth!

If Pentangeli had been killed or in custody, Rosato brothers, Roth's ally would try to take over Pentangeli's Corleone operations in New York as Roth intended

“If Michael had been killed in the Tahoe shooting, Pentangeli would have been blamed”
Pentangeli could not have known Michael's exact response for certain that Michael would side with the Rosato brothers / Roth

How can Pentangeli be blamed?
Pentangeli would have had no time to arrange the Tahoe shooting unless Pentangeli anticipated Michael's response and came prepared which seems highly unlikely especially as Pentangeli “I don't have your brain -- uh -- for big deals”

I think Tom would probably have taken the vice-presidency of the house and hotels offer and moved his wife, his family, his mistress, them all to Las Vegas even before Michael's funeral!

“Rocco and Neri were forceful and ruthless, but probably in the dark politically”
If Rocco and Neri had the stomach to fight for Pentangeli's Corleone operations in New York and came to some sort of joint partnership or similar, I think they would have been able to pull it off as they were always in the muscle-end

However I believe, Corleone casino holdings is probably beyond Rocco and Neri unless Tom opted to 'rescue' the business eg: increase the kick backs, more Nevada junkets etc.

If Tom, Neri and Rocco can figure a way to work together, keeping Fredo as a figurehead? “difficult not impossible” to try to keep the Corleone casino holdings going

Roth can't finger Fredo without incriminating Roth in the process

Then again without “always knew you were too smart to let any of them beat you” Michael's political and business connections, Tom in spite of “There's a lot I can't tell you” the smart lawyer that he is, might find a Khartoum, a girl with no family as if she never existed....oh wait! Tom is not a wartime consiglieri

As you say, Turnbull “All Roth would have to do would be to sit back and let them fight each other then pick up the Corleone casino holdings

Roth may not even bother with Fredo as Roth would have no further use of Fredo

And nobody, figures out it was Roth all along