I would classify the Navigate command as being more of an accessibility fallback then what we intended as the primary trigger for those that use this frequently.
You’ll find the methods listed in §13.1.1, Locking the Inspector, but in short: just right-click on the inspector tab bar area. 
The visible button was removed because this is not a very commonly used feature, and it was confusing those were making its usage more common by sheer accident and then wondering why the inspector didn’t work right.
I just added a question about truly locking the editor so that the ‘Show previous document viewed.’ and ‘Show next document viewed.’ carets are prevented from working - is that possible?
No, the rationale of the lock button is not to protect you from yourself, so to speak, but to stop all forms of passive navigation that would change the contents of the editor by merely clicking on things in other editors, or the binder. It’s not going to stop you from clicking a button, or using a hyperlink, or anything that could be interpreted as a deliberate attempt to navigate away from what you’re doing. It used to work that way, ages ago, and it was quite frustrating because most often when you click a button you really want the button to be clicked, and not have to wonder why nothing happened, and then remember why, and then turn off the lock, and then click the button.
(And yes, there are bugs in the complete implementation of that rationale, and you spotted one of them. It makes no sense to block you from being able to jump to the next document in the list, but not block you from jumping to the last thing you looked at.)