Almost everything that is a blade is designed to be curved or slanted, yes.

Many are. Scissors, drill bits, even the way most people use a kitchen knife, they put as much force onto as small of area as possible. At certain differences in scale it won't matter (razor blade vs a single hair), but how a cut is performed is definitely part of the design process.

Only those that go in frames. The angle is applied while using proper cutting technique for knives. A flat blade would also go dull rather quickly, as only one portion of the blade would be doing all the cutting (well, tearing, a straight drop with a flat blade would be like trying to chop a tomato rather than slicing it).

They are. Except for Japanese style knives, all knives have a slant.

The Japanese knives are only sharpened with a single edge, however. So they in a profile view look like a tiny guillotine.

Like a mandolin slicer?

chopping vegetables with a chef's knife applies the same principle