Following a lacklustre opening 45 minutes, PSG powered clear as they have done in the title race, with the excellent Angel Di Maria and Kylian Mbappe on target before the latter laid on Cavani's 20th of the season in all competitions.
Presnel Kimpembe's fabulous pass sent Di Maria scampering into space down the left in the seventh minute and his cross was nodded home by Cavani.
Julian Draxler took evasive action to allow Cavani to finish and should have added his own name to the scoresheet in the 18th minute after Di Maria and Mbappe worked space in the area.
That miss meant Niang was able to slide in a Rennes equaliser from Hamari Traore's cross, although the goalscorer was incredibly fortunate to retain a booking following a VAR review of his dreadful lunge on Kehrer.
Juan Bernat blazed over from Di Maria's lay-off early in the second period before the Argentina winger was again the provider for Draxler to drill too close to Rennes goalkeeper Tomas Koubek.
That seemingly convinced Di Maria to take matters into his own hands with an hour played, dinking a delicate finish over Koubek having charged onto Thiago Silva's raking ball.
Mbappe slotted in his 18th Ligue 1 goal of the season from Draxler's pass six minutes later – Rennes midfielder Clement Grenier having coughed up possession.
Draxler and Mbappe picked apart the disintegrating away defence in training-ground style for Cavani to notch his 71st-minute second as their seemingly unstoppable march to a sixth league title in seven seasons continues.
What does it mean? Rough treatment for PSG players and VAR in the spotlight.
At the end of a week when Neymar was left stricken by a robust challenge from Strasbourg midfielder Moataz Zemzemi, head coach Thomas Tuchel will have been relieved to see Kehrer walk away unscathed from Niang's tackle. The PSG boss was understandably frustrated to see his young defender on the receiving end of a foul that, while seemingly not malicious, could have caused serious injury. Why referee Karim Abed neglected to upgrade his call to a red card after looking at the wince-inducing television images is, frankly, anyone's guess.
Di Maria fills the Neymar void
Compensating for Neymar's many gifts is no easy task but Di Maria showed numerous flashes of his brilliant best, featuring at the heart of most of PSG's best play. He would no doubt love to retain such form when his return to Old Trafford comes around next month.
Ben Arfa unable to land a blow on old employers
However, returns to clubs where events did not go as planned are not always acts of personal vindication. Di Maria need only ask Hatem Ben Arfa, the one-time PSG outcast who operated on the margins of this game and dragged his only half-chance wide.