After a rather painful conversation in the comments of this question, I realize that I asked the wrong question, expanded on it badly, and made edits that didn't really help. Should I repost a new question, or entirely change the title and contents of the original?
As best I can tell, your "real" question just seems to be a clearer version of the question you ended up posting, rather than a fundamental change to the actual question. That puts it well into "Edit" territory in my book.
As I read it, your use of the word "explicit" was a weak choice, since the Shi'ite interpretation is that it was "explicit"; a fact that you were well aware of and lampshaded in the question itself ("One group says Muhammad was explicit, another group says Muhammad deliberately left the matter to the will of the Umma."). I would've gone with something more along the lines of "clear-cut" or "unambiguous", but otherwise the question itself seems fundamentally sound: You want to know why this isn't an issue in which there was absolutely no doubt among everybody concerned, and are looking for secular resources which examine it.
(of course, if i'm wrong in the above interpretation, then perhaps your question does need significant rework…)
I'm not convinced the comments thereon were remotely constructive or based on any actual misunderstanding, given that they seemed intent on bringing in exactly the same sectarian arguments you
explicitly clearly and unambiguously weren't interested in, even in spite you repeatedly saying you weren't interested in them.