Wow - that really would be a masterful plan by Roth! I do believe, however, that things are rather simple.

Michael feigned his desire to have Pantangeli killed in front of Roth. Roth knew Pantangeli would be meeting with the Rosatos, and Roth tried to make sure the meeting wasn't a trick (remember the phone call to Fredo in the morning?). Roth believed Michael was actually sending Pantangeli to the slaughter. Roth then likely informed the Rosatos that hit was OK'd by Michael. The infamous "line" by Rosato, even if ad-libbed, represented the Rosatos actual perception of the chain of events, and judging by their sleazy nature, was probably a little extra insult to injury for their enemy Pantangeli. In any case, the line was not articulated as some design for some future, grandiose, and unforseeable scheme. Rather, the line was a logical articulation based on the current understanding of the parties at the time, and, regardless of how it came about (via 'ad-lib or design), it was accurate. The fact that it may have lowered Roth's burden of turning Frankie does not mean the line was a part of any pre-pondered design; Roth simply made the best of the circumstances as they unfolded against his enemy. He played them 'beautifully'.