Exactly how customizable is the Rockbox UI? (Sansa Clip+; v3.13)

[MindNarrative]

I'm still getting used to not using an iPod for the first time since ages, and while there's few particularly significant *problems*, there's some stuff that's way less convenient than the iPod's UI. Here's some stuff I want to do:

• Changing around the order of the menu items in as many places as I can
• Selecting and playing a song from a db/playlist should go straight to the Now Playing screen instead of the main menu
• A setting or menu behavior that lets you keep a selected song static at the top of a dynamically shuffled playlist, so if I don't like song #2 I can quickly reshuffle and retry
• Lastly, I want to set it so that it pauses upon removal of headphones (super long shot), or so that it automatically goes to sleep after playing music for x minutes without button presses

[saratoga]

I think #1 is already the case, #2 cannot be done in current builds, and #3 works on devices that have the hardware required (clip series do not).

[MindNarrative]

Wait, there were four things, I screwed up the formatting. And yeah, I've found out since that the headphone detection is nonexistent.

But which one is #2???

[[Saint]]

I really hate to be "that guy", but, this question could have been answered entirely by consulting our fine manual.

• Changing around the order of the menu items in as many places as I can

This can be accomplished in the main menu only. Unless of course you want to compile your own build from source, then you can do as you please (but not exactly trivially).
See: 13.1.1  Customising The Main Menu

• Selecting and playing a song from a db/playlist should go straight to the Now Playing screen instead of the main menu

This is already the case.

• A setting or menu behavior that lets you keep a selected song static at the top of a dynamically shuffled playlist, so if I don't like song #2 I can quickly reshuffle and retry

Nope.

• Lastly, I want to set it so that it pauses upon removal of headphones (super long shot), or so that it automatically goes to sleep after playing music for x minutes without button presses

As noted in a prior response your device does not support headphone detection. A sleep timer, however, is present.
See: 8.6.3  Sleep Timer


[Saint]