Are these pants dressy enough for an office?

Yes for most workplaces. The wide leg and black color read as trousers, not leggings. The elastic waist makes them more casual than a tailored pair, so they suit business-casual settings best.

How does the elastic waist fit?

It is a pull-on waist with stretch, so it sits high and adjusts to the body rather than fastening with a zip. Size for your hip and waist measurement; the stretch gives some leeway.

What shoes work with wide-leg pants?

Sneakers for daytime and heels for evening, as the design intends. Keep the hem clear of the floor; a slight break over the shoe looks best.

How do these compare with leggings?

Same pull-on ease, but the wide leg and high rise give a looser, more tailored shape. They are less form-fitting and read as trousers rather than athletic wear.