I like this explanation, with the caveat that I think your line-up is a little skewed. Proto and Challenger, yes, I agree that those are the two "top" brands. But bellow them should be shifted to put P&C below Proto in the Professional brands, while Fleet should be moved over to the consumer side, albeit a different marketing version of Challenger, or possible between Challenger and Penens.
The reason for this is that Proto and P&C are the same down the line; ratchets, wrenches, sockets, pullers, etc. While Challenger and Fleet use the same ratchets, which are different than the classic 5442. These are descendants of the WF machined through type. And, lastly, those two brands, along with Penens, shifted to the round head ratchets, while Proto and P&C did not.
My reasoning with regard to the proposed arrangement is focused primarily on the marketing aspect, which I believe is the fundamental consideration in unraveling the Plomb/Proto menagerie with the brands originally inquired about by the OP. The emphasis you've made on how many tools are identical or different across the brands is once again focusing on the tools themselves, which does nothing to help understand the branding. Focusing on the similarities between the tools themselves is a well-worn path that not only fails to shed any light on the purpose behind the different brands, but is also the very approach that has led to the ongoing difficulty in sorting out these brands (and, if I may, many "skewed" ideas). Calling out the many tools identical across the brands, while quite intriguing and a fascinating aspect of tool collecting, has proven to be largely irrelevant and even futile in the effort to determine the rhyme and reason behind the multiple branding. I propose that the only hope to sort this all out is in focusing on the marketing strategy behind the branding instead of how many tools are the same or different, which has led us nowhere. After all, if so many of the tools are identical, the only
meaningful distinction is in the brands themselves, not in the tools. It is therefore purely a question of marketing.
For example, you've pointed out that period Proto and P&C tools are virtually "the same down the line." That's my very point! If the tools are identical, what is the purpose for two different brands? If the tools are the same, couldn't they all be sold under one brand instead? Looking at the tools themselves sheds zero light on this question. In order to sort this out, we need to at least temporarily step out of the traditional mindset of focusing on the physical tools themselves and instead pay attention primarily to the marketing strategy, which is, after all, the only remaining viable factor to consider that can distinguish the difference in the many instances where brands like Proto and P&C are otherwise identical.
When it comes to P&C, there's another aspect to consider. The reason why OP's question regarding Proto, Challenger, Fleet and Penens is so core to this issue is that Plomb actually
created these latter three brands. Plomb created these brands, not merely from a corporate viewpoint to establish separate entities for legal or logistical purposes, but also marketed these tools to the public under these same brands. So this begs the question: why did Plomb create these different brand names to sell its tools under? There is only one plausible answer: marketing. So here again, comparing the physical tools themselves tells us nothing about the purpose behind the various brands they were sold under.
In this light, it's easy to see that P&C is a different animal than Fleet, Challenger and Penens. Plomb did not create the P&C brand--P&C had an established brand with both an existing name recognition and a following. The fact that so many of the P&C tools became identical to Proto is irrelevant to the question of branding--P&C had a different audience, and so Plomb's decision to perpetuate P&C as a distinct brand was entirely a matter of marketing. The P&C history site makes mention of the distinct ways in which the P&C brand was marketed, and even received a national award in 1963 from the National Wholesale Hardware Association for the company’s “contribution to the improvement of manufacturer wholesaler retailer relationship.” This mention of improvements to the manufacturer-wholesaler-retailer relationship--what is sometimes known as the distribution
channel--is helpful in realizing that P&C was being marketed in its own unique way as an independent entity with its own distribution channel strategy distinct from the other Plomb brands.
For this reason, I did not include P&C in considering the consumer hierarchy of Challenger (top tier) and Penens (second tier) because P&C was not a part of the marketing strategy behind Plomb's creation of the Fleet, Challenger and Penens brands--P&C was maintained and marketed as its own, distinct brand. As is often the case with multiple brands from a single company, Plomb in a sense was running its own competition where P&C was more of a direct competitor to its Proto brand rather than being marketed to retailers as any part of a good-better-best relationship to be included with Plomb's created Challenger/Penens pedigree. One might come to this three-tier conclusion by focusing on the physical tools, but not when the focus is strictly on the marketing that gave rise to these different brands.
Again, trying to reckon the order behind the various brands in question based on studying the relationship between the physical tools has proven fruitless, which is the strongest clue that the key in unraveling the mystery is not in the physical tools themselves. A much clearer order emerges when we focus on the marketing aspect, the logic of which then becomes quite evident. That's because the marketing was the very purpose behind the existence of these multiple brands to begin with.