I can't say whether you should return to your old employer. But you should absolutely get out of rancid-trash-culture place ASAP, and if returning to your old employer accelerates that, and you're okay there, why not, at least for now?
But you don't owe either employer any particular length of stay. You know they'd drop you like a hot potato if they had a business reason to. Sure, go back to the old place -- and ask for everything you want, but I wouldn't make ultimatums, just ask and see what offer they make, and whether it satisfies you. And stay as long as you're happy there, and leave again if you have reason to.
And -- no, it's not wrong to ask for a higher salary or higher equity share. They can always say no. Most likely they'll counteroffer. There's no immorality to asking for what you want, or even getting it. And there's no immorality to walking away if they don't offer you enough. It's all just business negotiation, and morality doesn't really enter into it, at least on your side. Companies generally have more power than individuals, so any moral implications would be on their side.
Honestly, I think the most important thing is getting the hell out of a toxic workplace. That can really mess you up.