I am still pretty sure that you don't really need 8 cores, but it's your money.
I hope you thought about B450 vs B550.
Max II is a bit newer so slightly higher chance of the 5800X working out of the box, kind of irrelevant though since they can both be updated without a CPU installed, which is one of the reasons why I recommend them. Also HDMI 2.1, which you obviously can't use.
I haven't checked specific temperatures, might work, you can obviously just set the fans to 100%, not sure how much voltage PBO will try in that case though, but I doubt it would heat up too much.
No idea if you'd need dual tower for your specific goals. They might be worse for RAM compatibility and two fans are usually louder than one (especially at low loads they're not that much better that lower rpm could compensate for that). Noctua might have the nicer mounting system but it's always hard to justify the NH-U14S when a dual tower Dark Rock Pro 4 only costs ~7€ more.
Nah, this is about single vs dual rank RAM. Zen likes having two ranks per memory channel which means either 4 single rank DIMMs or two dual rank DIMMs.