I think I read somewhere that it was Kelly's name on the rent book, but can't remember where.
Yes, I imagine that McCarthy would just try to get his rent from anyone he could, so Barnett's continuing appearances in the Court suggest to me that somehow McCarthy had been sweetened. One theory (more a piece of speculation really) is that McCarthy was waiting to see if Barnett could pay in kind, from his new job. I suspect Barnett had been pinching fish for McCarthy from the market. That doesn't make him a murderer, of course, but it might explain why he lost his licence. After that happened, Barnett's part-time work in fruit gave him less chance of having anything to give McCarthy.
I wish we had all McCarthy's rent books from all his tenants, but obviously that's never going to happen. That would give us an idea of how McCarthy dealt with people in arrears in general. I imagine that landlords always had the dilemma, do I kick out this person, write off the arrears and re-let the room, or do I let them stay in the hope they can get back on their feet and gradually pay off the deficit? I suppose it would have depended on the landlord's assessment of each individual tenant.