There is no overarching blanket concept in Deleuze’s philosophy. In several of my posts I am merely using the concept of multiplicity strategically in order to contest the false unity that is often projected onto Deleuze’s work in progress by seeing it as simply reducible to a philosophy of difference.
This is also one way to defend Deleuze from Laruelle’s critiques, which are based on this monistic reduction of Deleuze’s pluralism to a philosophy of difference. I would add that I also refuse the reductive overcoding by means of the “fold”, reading it retrospectively as the major concept of Deleuze’s philosophy.
On the question of Deleuze’s third synthesis of time, I accept its futurality but I reject its characterisation as “return”. The attempt to mitigate its incoherence by the denomination “intensive” return is a cop out. Deleuze in his later works keeps the futurality (e.g. in his evocation of a “people to come”) but drops the “return” and de-emphasises “difference”, and this is an improvement.
On the question of the treatment of the “eternal return” in RHIZOME: in the introduction to A THOUSAND PLATEAUS, Deleuze and Guattari make a distinction between the figure of the a-centric non-unitary rhizome and figures that apparently renounce unity but that re-constitute it at a higher level. This is the heart of their analysis of the “fascicular” system as opposed to the rhizome.
In this book Deleuze and Guattari analyse the “eternal return” as belonging to the the fascicular mode rather than to the rhizome. Similarly, they consider Burroughs’ “cut-up” as being fascicular, involving a “supplementary dimension” of folding where a spiritual unity persists. In DIALOGUES Deleuze contrasts Burroughs’ cut-up technique with his own “pick-up” method, which involves consistences arising out of contingent encounters.