(a) First note the the cross section is "arbitrary". In particular, you may not not take it to be a circle. Also your interior point must be arbitrary: not on the z-axis, or any other axis.

Second. notice that, with Jackson's sign conventions form P5.1, the solid angle subtended by the entire solenoid is zero at any interior point. Thus you will need to break it up into two parts: one due to the part of the solenoid above your point and one due to the part below. Start with the solid angle due to one loop, (as in P 5.1) and expand 1/|x-x'| in Bessel functions. Leave the result in integral form, take the gradient, and then evaluate the integrals.

(b) Here the cross-section IS circular, but you have to account for the fact that the wire is a helix. I used the Biot-Savart law to get this result.