To answer your a)... Because most users don't think about their browser and hence use whatever the default is. OEMs will likely never bundle Firefox, but there is a chance that they'll bundle Chrome since it has Google's name behind it.

Hence, Chrome rescues users (developers?) from IE.

As for your b)... I agree totally. Chrome is nothing but an application delivery platform for Google apps that is disguised as a browser. Which is all the more motivation for Google to pursue bundling deals with OEMs.