How can I rename the default album from "Wall Posts" to "Default"?
This is not recommended, the way the code stands today. However, we are rapidly rewriting a lot of code in the background. There are talks of BuddyPress adding a default album in the core that we can leverage in the near future.
So, as of now, it'd be better to wait and watch, for a while. In case, it is really that critical for your implementation, you'd have to make changes in your code and maintain these over upgrades. This is not recommended unless you have a strong coding background.