As Malloy's article explains, neither Groves nor the scientists involved in the use of the weapons thought radiation sickness would be a major problem for the victims of the bomb. Why not? Because 1. they thought that anyone who was close enough to be in the zone of radiation sickness would be killed by the blast and fire effects (which is not crazy — compare the relative size of the radiation versus other effects — but it was wrong), and 2. that by detonating it at a high altitude, they would avoid significant fallout contamination (which they were right about). Now the real world turns out to be more complex, and there were significant (maybe 15% or so of the total) radiation casualties, and it is harder to reduce them than they thought. But they do seem to have genuinely not thought it would be a big problem.

Was there an intentional effort by General Groves and the rest of the Manhattan Project team to keep information about radiation sickness from those deciding whether to drop atomic bombs on Japan?

I just want to note that your initial question contains an erroneous understanding in it. There was no real "decision" about whether the drop the atomic bombs at the "top." It was assumed by those on the project it would be used, and the decisions that led to it were being made by people like Groves, Oppenheimer, and others. So focusing on whether the President understood is sort of a red herring. For another good Malloy article that discusses this, see Sean L. Malloy "‘The rules of civilized warfare’: Scientists, soldiers, civilians, and American nuclear targeting, 1940-1945," Journal of Strategic Studies 30, no. 3 (2007), 475-512.

Was FDR was informed of the possibilities of radiation sickness before his death?

No, but he wasn't informed about much, so I'm not sure what difference that would make. None of the reports to the President prior to Trinity were very detailed about the power of the bomb — they made estimates of the kilotonnage equivalent, but nothing about the detailed effects. This goes for Truman as well. Radiation as far as I know never made it into the reports. Again, I don't think that's because it was being "kept" from the President; it's because the scientists (wrongly) thought it wasn't worth worrying about.

When Groves told Congress that radiation was "a very pleasant way to die" did he believe it?

Probably not. But this was in the postwar period when Groves was under assault by Congress, scientists, and everyone else for the supposedly roughshod way he ran the Manhattan Project. In this case, he was backing down from his initial assertion that radiation hadn't mattered at all, replacing it with the lie that it really wasn't so bad.

Did knowledge of radiation sickness impact US or Allied public opinion in any significant way?

I think it is clear that radiation effects did shape US public opinion about the nature of the bombs and their use. It certainly is the feature that became separated out as "special" in postwar discussions. How much of that is based on a realistic understanding of radiation sickness, and the somewhat more fantastical visions of it, is hard to say. But radiation certainly did change how the US viewed the bombs (and still does). On this, see Spencer Weart, The Rise of Nuclear Fear. Some of that comes from later things, though, like Operation Crossroads and John Hersey's Hiroshima, to say nothing of the Castle Bravo accident.

The other book I would recommend if you are interested in probing deeper is Barton Hacker's The Dragon's Tail: Radiation Safety in the Manhattan Project, 1942-1946. It's very useful in constructing what the scientists and others knew about radiation. They did think about it a lot — but in the context of operational safety, not victims of the bomb (as Malloy argues).