I think Niall would have continued working for Paddy. It’s the life he knows and he wouldn’t think he could do much different. Paddy would give him some time off to recuperate and I think Niall would have been easily convinced to do so back at Harrys mums. Niall would probably think it would give Anne and Robin more time to convince Harry to change his life. Gemma would probably come out too and maybe over an early cuppa when Gemma had taken Harry out to the shops or something, Anne would question Niall. Niall would tell her that he’d never ask Harry to stay, but it’s up to Harry if he’ll go on his own. He won’t push him again. It has to be Harrys decision and he’ll do everything possible to keep Harry safe if he decides his life with Niall is what he wants. It’s not exactly the answer Anne is looking for but she can see how much they love each other, how much they need each other, so she’ll let it go.
Eventually they go back and Niall continues on as paddys driver and they move back into their flat as shitty as it is.
Harry hides things from Niall. A few months in he has a break down, never having let himself really go through what happened with Niall because he was so focused on making sure Niall was fine. He has nightmares and panic attacks, won’t let himself sleep when Niall’s at home, pretends to pass out on the sofa and in the end just leaves a lot. He avoids Niall more and more because he thinks Niall will make him go if he sees how badly Harry is coping and as much as he doesn’t want that he can’t be around Niall either because then Niall will know. It gets to a point where they have a yelling match about it just before Niall has to leave for work and once he’s gone Harry ghosts.
Louis is the one that finds him, drugged out and fucked up in some hovel and Louis takes him back to his mums to sober up. Harry begs him not to tell Niall, and Louis doesn’t because he’s sort of got to know them all since they’ve got back. Has known of Niall anyway and through his mum and a quiet word with paddy, they get Harry someone to talk to all under paddys money. Turns out his soft spot for Niall extends to Harry for the way he coped during the shit that went down and how he’s looked after Niall. Kept him level headed.
Eventually Harry and Niall run into each other somewhere and Niall chases Harry down, makes him talk because it hurt when Harry left. As much as he thought he could cope with Harry not choosing him, he hasn’t. Drinks too much, gets into too many fights that he shouldn’t (has the tail end of a black eye that Harry can see and his knuckles are ragged and bruised). Harry tell Niall EVERYTHING and they decide to work it out, Niall going to paddy and ending up not quitting as such, but taking over one of the LIC pubs that isn’t in such a bad area and has a much larger, nicer flat above it for Harry and Niall to use.
Harry still has bad days and Niall still has to do things he can’t talk to Harry about for Paddy but they make It work