What is interesting about this message is that I don't think either Nick or Aaron come across as very honest in it. IMO they were both snaking each other in terms of their intentions with April. Nick is probably not actually interested in saving Aaron's marriage to April and Aaron's simping for April probably doesn't quite reflect where he actually was at that moment.
100% Nick was triangulating Aaron and April - it's a classic narc move. Position yourself between two (or more) people, play good guy with both, gain each's trust and dependence, become the keeper of all the info, share or lie as it suits your interests.
In this case, Nick wanted his wild affair with April but not to have her as his responsibility or obligation. So keeping Aaron hopeful about his chances with April, at least to the house purchase, was important. He perceived Aaron as weak and controllable and was confident that even if they stayed together he'd still have April any time he wanted. And them staying together meant April was still at arm's length and dumpable, and he could avoid any serious demands or (unmanageable) drama from her. It would be on Aaron to take formal care of her. So Nick could just be the feel-good lover she was hooked on while her husband, though embroiled with Kayla, would stay as April's second choice. He also knew Kayla would never leave him for Aaron. So all of this was Nick positioning everyone where he wanted them - under his control and thumb. He would be both women's top choice, and Aaron would be his beta cuck in every way. Nick would be king of the 'cule.
...so when Aaron grew a spine (or had an epiphany...or whatever) and noped out (or April lost her mind for Nick and rejected reconciliation and Aaron accepted it despite Nick's urgings), it not only wounded Nick's sad little narc soul but also threw a giant wrench in his scheme. Now April was his problem and dependent; no one was buying the house so there was that problem to solve (a big one, apparently, as he needed the money); and the lack of balance from the quad becoming a trio would lead to Kayla's increasing dissatisfaction - another unpleasant headache for him. (The dissatisfaction would have happened eventually even if the arrests hadn't occurred, bc it's one thing for your husband to be madly in love and spending time with another woman when you, too, are in an exciting affair, but it's another when all you've got is Star Trek and 5 kids to ignore while your husband is down the road coked out of his mind with a girl with a non-"destroyed" body playing perfect, devoted, sexpot lover.
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Another thought: Nick now claims Aaron was "abusive" and "non-consensual." Hey, April, why did your super-lover who cared so much about you encourage your abusive husband to try to be with you? Did he tell you Aaron was a bad man even while telling Aaron to fight for you? Or did he try to push you toward Aaron as well? Either way, he was doing something to encourage your "abuse" to continue. Remember that. Engrave it on your heart, and develop better discernment.
Or, now that I've read
@HenryRollinsLiar 's comment, yeah, fix yourself, too. Too damn many cluster Bs to count here.
Nick is a narc and he's just had a massive ego injury,
Everything you said, too.