Oh, I just realised, the mehndi is in Lahore right? Then I don’t see the problem.
But even if you can’t attend that, you can still go with the Barat for that day and attend the Walima. Or at least just the Walima, that’s the event from your side right? As you said, you will regret it if you don’t.
Anyways, I reckon even 5 days is ample time in Lahore. You won’t believe how much I got done in 2 days, lol. Great ready for some 10 hour shopping days though!