Does the C-channel section have a uniform wall thickness, or are the short side ribs tapered?

Midsurface only works on parallel faces, so you can't capture tapered ribs. You can convert the channel to a Beam model, which will replace the solid body with a beam with the correct cross-sectional area properties.  Beam elements are a higher level of idealization than Shell elements.  If you want to use Shell elements, you won't use the Midsurface button, you will pick the three outside faces of the C-channel and do a Ctrl-C, Ctrl-V to create the surfaces to mesh.  You will manually assign a thickness to those surfaces in Mechanical. The tapered ribs will be assign a constant wall thickness, like the average between the base and the tip of the rib.

You can assign the 10 MPa internal pressure in a Static Structural model and feed the Solution cell of that into the Setup cell of the Modal analysis. This will cause the stiffening that the pressure causes to the pipe to be taken into account.