With three minutes to go, a mistake by Boyce allowed substitute Danny Welbeck to square for Van Persie, who gratefully clipped his second and United's fourth.
Kris Boyd scored the first of the goals from a free-kick that seemed a generous award, while a dreadful error from defender Darren Barr allowed substitute Steven Naismith to double the lead in the closing minutes.
A well-worked move allowed substitute Kevin Gameiro to find Aleix Vidal down the right who cut the ball back for Iborra to finish.
Arsenal's defence allowed substitute Sandro too much time to cross and when the loose ball fell to Walker, his shot carried pace and swerve but Szczesny should not have been deceived to let the ball evade his grasp.
The visitors claimed their solitary try when Ross Samson's quickly taken free-kick allowed substitute Joseph to race down the flank and exchange passes with Adam Thompstone before powering past two defenders for the score.
Leeds' hopes of a comeback were undermined by their own farcical defending, however, as indecision between Silvestri and Giuseppe Bellusci allowed substitute Jones to slot into an empty net and ease Cardiff's nerves.
A rush of blood to Williams' head allowed substitute Forrest to round the goalkeeper and set up Teemu Pukki in front of goal, but the Finland striker hesitated as McGivern blocked the low effort.
