The reason she'd gotten into the mess in the first place is that she expected special treatment from people she'd witlessly raised to treat her like a God-damned doormat owing to her revealing herself to have all the firmness of a bowl of melted Jell-O. It would never occur to her offsprings that she deserved an inside space because, as we've seen, she can't get it through her fat head that they cannot read her empty mind no matter how sincere she is.
There are three reasons that she bellowed at John. The first, of course, is her perception that he wasn't taking her concerns seriously. In her alleged mind, what he was supposed to happen was that the children would remember who purportedly raised them and let her have the inside space she wanted. This sort of thinking is similar to a theme from the comic strip Cathy that I find irritating. Every so often, Guisewite had her avatar smile indulgently at her foolish husband and his baffling habit of offering solutions to the woes that she and her friends will do nothing about. What the silly man doesn't understand is that they know what they have to do but they're just so hampered by the Disnified mindset that celebrates smiling passivity as the highest feminine virtue, they can do little more than agree how horrible the problem that they wish they'd been raised to conquer is. Where Johnston differs from Guisewite is that the latter realizes that this is a problem because of her remembering how deeply messed up her characters are. This translates to Elly storming around acting pointlessly upset because she thinks that when people offer to help her, they're either showing a lack of faith in her ability to do things for herself or mocking her for worrying about nothing important.
The second, of course, is her need to hold grudges and remember slights. Since if a problem goes away forever, she's not going to be able to hold on to the only thing that makes her life worth anything to her, it doesn't take a genius to realize that she doesn't actually want her life to get better lest she have nothing to do with her time.
The last, of course, is that she doesn't want to have to admit that she should have set ground rules in the first place. After all, if you want to make her your enemy, the best way to go about it is to hint that she made a mistake at some point. Not even John is stupid enough to tell Elly that having Mike stay at their place for months on end is the wrong thing to do for the wrong reason because he'd exploit it as shamelessly as he did; offering band-aid-on-a-trainwreck half-measures is the best he can do.