Time scaling of discrete-time sequences and the DTFT
The DTFT of some sequence $x(an)$ is discussed in the book you mention, but rather indirectly.

It's easy to see that $x(an)$ is a decimated sampled version of $x(n)$. This means that we take $x(n)$, we sample it with sampling period $a$, and then we decimate it (i.e. we take every $a$-th sample).

You can derive the DTFT of $x(an)$ knowing the DTFT of $x(n)$ with this in mind. If you have a copy of Oppenheim's Signals and Systems, check the section Sampling of discrete-time signals. Everything is explained there to answer your first question.