See that’s the problem with using categories which may not be applicable. Applying the nomenclature of “literal” to the difference between slavers and anti-slavers is in my view misplaced. After all, scripture literally says that if you can seek your freedom as a slave, you ought to do so. And it says that in Christ there is no slave nor freeman, which implies a certain type of equality in humanity… this part the so-called literal readers chose to ignore. Nor does scripture anywhere literally mention such a thing as a “negro”, nor does it equate “negro” with the curse of Ham or with slavery. So they have simply made the equation that Ham is black or african, but there is no literal proof of that. In fact, it is Canaan who is mentioned in connection with slavery, and it would be logical to suppose that Canaan inhabited or was connected with the land of Canaan, not with black africans. Interesting also that the King James uses “servant of servants” which is the translation most slavers had at the time. Interesting also how the Jacob talks about Issachar being a bland of slaves (KJV) or forced labor (NIV). But this must have escaped the attention of the slavers. Point is, favoring slavery had nothing to do with literalness, but rather with selective reading.
Other than that, you have raised some good points. However, YEC people maintain that in general there is lots of evidence for a global flood, in terms of the fact that almost all sediments were laid down by water, not by wind, that water dwelling organisms fossils are found all over the world including the highest peaks on earth, that fossils can only be created by rapid burial, not by slow sedimentation, that erosion of major canyons happened quickly, not slowly over eons of time, that for sedimentary layers to fold in mountains required them to be pliable and thus not yet hardened at the time of folding, that fossils are generally found in groups that make sense from a flood perspective, that there is a mixing of many types of organisms in many places, and yet that fossils are sorted to some extent as would be expected when impacted by a flood, and that organisms including dinosaurs and mammal fossils are found in groups which apparently experienced multiple disaster at time of death. Then there are the polystrate fossils which also indicates rapid burial and deposition of many layers in a relatively short period of time.
As you say, if we accept the fall of man into sin in the garden of Eden, but deny the global flood, a creation- fall -redemption structure could be maintained. However, the reliability of scripture would be in question. In addition the need for justice under God’s demands and expectations would be diminished, since the flood is the great teacher of the universal rebellion of man, and the normal justice that would be expected, for which God provided a substitute atonement. A local flood simply would not teach that truth.
I don’t think anyone is covering up evidence. They are not seeing what the evidence points to. And this is obvious from the many false starts and false conclusions that evolutionary theory has brought forward over the years, which have eventually been changed.
There is however indeed an explicable motive, which is a motive derived from a naturalistic and materialistic philosophy, that even if evidence for a flood were to be seen, the cause could not be explained in purely naturalistic terms. In addition, while you might argue that they could say a global flood happened but Noah, sin, and rainbows are pure mythology, it would be difficult to admit or even to confront the fact that scriptural records had concluded a global flood long before geologists felt compelled to admit it.
This by itself does not prove that a global flood happened, or that eyes have become so blind as to not see it. Nevertheless, it is as valid as arguing the creationist motive.