The Laplace transform technique is an established technique that is related to the characteristic equation method, is systematic, and can handle inhomogenous equations naturally without having to guess the form of the particular solution. It does not use undetermined coefficients or variation...
You basically got it. Your equation (7) is setting a constant P equal to a Fourier cosine series. The lowest order term in such a series is a constant, and you even correctly identified the value c_0=2P; if you take half that, this IS your \bar C_0 value. Working that back to your original...