You seem happy = You look happy = You appear happy?

[Mister Micawber]

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On paper, I see no difference in meaning whatsoever, except that seem can apply to other senses than sight.

No one can know in any case whether the other person is 'really' happy. In practice, tone of voice along with idiolect could imply other variations of confidence on the part of the speaker.
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber

[SkiIucK]

[Yankee]

Hi SkiIucK

I'd say whatsoever is an intensifier and can be used interchangeably with at all in negative sentences as well as in questions and IF-sentences:

I don't have any free time whatsoever/at all at the moment.

Did he have any idea whatsoever/at all what the source of the problem might be?

If you had any decency whatsoever/at all, you wouldn't treat her that way.

Amy
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Amy
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ESL teacher, translator, and a native speaker of American English

[Mister Micawber]

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[Mister Micawber]

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What ...is. I think.
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_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber