regarding the hagar and keturah
you are ignoring the plain meaning of a rabbi that says regarding keturah "zoh hagar - this is hagar". But ok, I understand you -have- to ignore the plain meaning if it contradicts what another rabbi says.. Since you assume that all those rabbis opinions are true, and it's as if they came from the same sane consistent rabbi.
and since you don't have ruach hakodesh, you rightly said, your explanation is a -possible- explanation.
Though, as rabbi gottlieb said, "there's always an if". One doesn't really know 100% who one's parents are. Maybe they went to the hospital, found a baby that looked a bit like them, they weren't the parents but they convinced you that they were. It is remotely possible that the world was created 1 second ago. Is it possible that G-d doesn't exist ? Rabbi Gottlieb said, yes! There's always an If.
so the question you would have to ask regarding the explanations you come up with , is how likely is it that they are correct?
There are hundreds of creative explanations that rabbis without ruach hakodesh could come up with. All different. Not all correct.
So since you ignore the pshat of the words of rashi and ramban or of 2 midrashim , on hagar and keturah. You ignore it In favour of these poetic explanations that reconcile the 2. Then you really don't know if you are correct, or the other hundred creative rabbis(without ruach hakodesh) , with different explanations, are correct. And since all the explanations coiuld differ, it may be that only 1/100 of them are correct. Or 1/1000 of them. Or none of them, since there are another million creative explanations nobody had thought of yet, and one of those was correct.
An explanation that is invented, may be implausible.
What If blue elephants exist above your head.. You can't prove they don't. Maybe they do, there's always an If. Or, they may be plausible, simple explanations. But there may be millions of different ones that one could invent..
With this method, the likelyhood of anybody understanding -anything- of any rabbi of that era, is very slim.
I hear this question. I don't think you'll like my answer but it's the answer you need.
The answer is that G-d was kind enough to bless this generation with a Torah Scholar that merited not only Ruah HaKodesh but also the level of prophesy (a topic we can discuss later on another thread).
In any case if you are looking for highly authoritative treatment that reconciles seeming incompatibilities throughout Shas,Medresh, Rishonim etc. I would highly recommend that you invest in "Likutei Sichos", "Sefer Hasichos" and then eventually Toras Menachem by Rabbi Menachem Mendel Schneerson (these can be purchased from Kehot Publications and many of the books are available in Hebrew...start with just one book if you can't afford the entire set..ask for a volume that has a lot of his "nigleh sichos").
Learn them with the footnotes provided and you will find more maginificant resolutions throughout Shas and Poskim then you ever dreamed possible. It will truly open your eyes to a whole new world of learning. I have not learned through all of these books myself. But I would venture to say that the resolutions to all these seeming contradictions between those of the "Ruach HaKodesh era" are resolved in these books implicitely if not explicitely.
The only downside is the Rebbe in certain talks often throws in kabbalistic terminology and if that ever holds you back from understanding I will gladly help translate those parts into plain english so you don't lose the flow.
I think if you start learning this stuff you'll never turn back.
If you're just looking for something easy in English I'd recommend the Gutnick Chumash with the Rebbe's commentary. Very watered down and not the Rebbe's exact words, but you can get a faint flavor for how he takes apart a Rashi, a Sforno, a Ramban and it looks like a total mess and then shows you the deeper meaning behind all of them and shows how they all make sense and ties them all together around a single point.
It''s something magnificent to behold.