To play devil’s advocate- about the oneg Shabbos issue, yes all learning is enjoyable for the Neshoma, but people like learning different areas. For someone who usually learns Chumash because that’s what they enjoy, but is “forced” to study for a Gemara test which he enjoys less… I could see that as lessening oneg Shabbos.
And hachanah- if you would be doing something else, but are only learning this because of the test, why wouldn’t that be hachanah? So, I have heard that if you are taking a nap on Shabbos because you want to stay up late, you are allowed to on Shabbos since it is inherently enjoyable, as long as you don’t say you are taking a nap for after Shabbos purposes. I guess that argument would apply to studying Torah on Shabbos.