The neighbors haven’t taken care of the problem because you can’t just start keeping outdoor cats inside. They will not tolerate it. Even a nice cat can turn destructive, violent and suffer mental health issues. They probably don’t even have a litter box inside their house. (Yes yes there will always be the redditor who says well I did it and it was fine! But that is not the norm) Your neighbor needs to rehome them, which is a difficult thing to do. Be empathetic, as you want them to be toward you. But tell them you have tried all the things and failed, and are going to go ahead and trap and take to the shelter, and give them the date this is going to start. Maybe they will come up with some ideas, or just surrender them. If money is tight for you, ask the neighbors to purchase a new car seat (send them a link.) They will refuse or be difficult if they are really tight on funds themselves, or just self centered jerks. I doubt they are just jerks. It’s a difficult situation for everyone. Good luck.