Answering in a different order:
r/frugal is not a great community because most of what they've said there is "penny save, pound foolish". What I had in my mind when I said that was on driving habits, for example, which is what I saw on MMM.
Your 3rd point - I did ask about it, but the question was buried underneath "Personal Milestone!" posts and Linked posts (not that popularity matters, but I only got one response from a person who gave their guess to the answer). I think it's been something that a lot of people have talked about indirectly when they discussed IRA vs. 401K, but nothing specifically about reverse rollovers. And, I think it would apply to more than a few people, especially if any of them are regular 1099 employees or receive any amount of "consulting" or regular income from a 1099.
Controlled groups- It's to establish whether an entity, regardless of business line, is in a parent/child relationship or brother/sister relationship. They were created to prevent companies or business owners from excluding groups of employees from company retirement plans.
Although it's a concept that affects businesses traditionally, it's relevant to individuals who have multiple sources of income, especially if some of your income comes from freelancing (which is thought of as a Sole Proprietorship when considering taxes) and/or you're a business owner or partner of multiple businesses. Basically, if your LLC or S-Corp is in the same control group as another LLC, S-Corp, or Sole/Partnered Proprietorship, it will affect how much you can put towards a SEP or solo 401K.
As an example, let's say you earned a good chunk of money freelancing as a foodie blogger, but your side business in software development, of which you're the sole proprietor, had a loss. You can't just put 25% of your blog's profits into your solo 401K, since both "companies" are in the same controlled group, even though they're in completely unrelated lines of business. On the other hand, let's assume that both of those ventures did spectacularly - the sum of 25% of the profits from each is over 53K. You can't put in that sum because they're considered the same employer (you). If they weren't in the same controlled group, then you could sock away that sum of over 53K.