The downloadable and online versions of the Rockbox Manual for your player are a good thing, along with the ctrl + f search command.
1. Rockbox Zip Manual p. 31 (Key lock): Home + Select.
2. Rockbox Zip Manual pp. 36 and 57: Play the directory you want to listen to, and set the player to shuffle. Alternatively, create a playlist of all your player’s music (a simple process), play that playlist and set the player to shuffle.
3. Rockbox Zip Manual p. 71: A multitude of startup options, including everything you asked for.
4. Rockbox Zip Manual p. 18 and prior pages: Updating Rockbox. Note that, as you correctly note, you currently have the latest “release version” of Rockbox, that is, the latest version of Rockbox that generally was deemed to be stable and “clean” enough as a whole (a major undertaking, as you probably can understand) to be considered the current “official” version of the operating system.
But as you might have seen at the Rockbox.org website, developers add changes and sometimes even features to Rockbox on a minute-by-minute basis–the “development builds.” You can read about the changes at the opening page of the Rockbox.org website in the “Code changes” section and the linked page–this shows you what changes have been made. And then you can determine if you would like to get the latest build of the operating system incorporating the changes. As a general matter, I have moved to a new developer’s build when I’ve seen that a new feature or enhancement that I like has been added or made (and I might wait a few days, to allow time for any bugs to be caught). I can’t recall ever having had an issue, and certainly nothing major.
The latest development build can be obtained through the Development Builds page and then installed manually (really, a simple enough process, covered in the manual): https://build.rockbox.org/. The Rockbox Utility also can be used to install the development build. https://www.rockbox.org/wiki/GraphicalInstall
Note that development builds can contain bugs. Having said that, in my experience, bugs are reported and corrected quickly–all is reported in a near-live basis in the Code changes section. And in my experience, bugs have tended to be just that–typically, minor issues, which then get corrected, promptly.
Finally, if you’re up to some some challenge, you actually can patch–that is, amend–the Rockbox code yourself, to make changes of your own desire. Needless-to-say, this requires more effort, but it is do-able, even for us mere mortals. The Rockbox Development Guide can give you an idea as to and instructions for the process. https://www.rockbox.org/wiki/DevelopmentGuide
Good luck, and enjoy!