I mean, the Joker's fighting ability and strength have never been consistent.
He once got into a fist-fight with Lex Luthor (literally a human being in peak physical condition with fairly extensive martial arts training) on an alien planet when all the villains got teleported there, and the fight was a draw.
Then in something else he'll flee like a bitch when confronted by someone else who theoretically shouldn't be as tough as Lex.
There's arguments that his insanity makes him a more deadly physical combatant than most people, and then there's times where he's got like assassin-tier knife skills.
I mean, changing their "attitude" is one way to say it, I think of it more as their brain getting bleached.
But either way, Harley had a gymnastic background, which is when she was more often than not used as the Joker's primary muscle because she could flip around and beat up Robin or Batgirl or whatever.
The reason the Joker would make her jump into the chemicals would be for fun. Either she'd die and he'd move on, or she'd live and probably end up just like him. He would essentially not care either way and fine with whatever consequence, because at the end of the day all he cares about is Batman.
But in the movie they had him jump in after her like a beta, which is not only stupid for the reason I mentioned in my previous post, but also because it's simply not in his character, or rather shouldn't be.
If the Joker "cares" about something that isn't fucking around with Batman, he's not really the Joker.
Whenever Harley Quinn fucks off to go lez out with Poison Ivy and/or Catwoman, Joker does his own thing and doesn't care.
If he fucks with the Suicide Squad because Harley's in it, he's not doing it because he's upset about Harley, and he's trying to "get" her, he's doing it because they ran afoul of him and he had nothing better do to.
