Well, given you've provided so much information..... I'd say it's because you didn't dance naked under the Summer Solstice while burning money.
I can lock/unlock paths in symbols and AI 16.0.1 with no problem.
Not exactly sure what you are asking.
To edit the paths in a symbol you have to double click the symbol to entere the symbol, or you can break the link to the symbol.
To lock other paths than your current selection do a select >> inverse priro to locking. Or try object >> lock >> other layers
This is for CS5. However, you can lock layers in a Symbol. That is not the question. The question pertains to locking all other layers/unlock all layers.
In symbol edit
Lock other layers in the layer window menu does not work, it is greyed out.
Object>Lock>Other layers is also greyed out
Why is this? Combined with the fact there doesn't appear to be a way to select all objects on a layer, this makes reasonably editing symbols almost impossible.
Clarify... are you referring to layers or paths... you've now posted both.
Symbols only contain 1 top-level layer, then as many sublayers as you desire. Sublayers are not the same as top-level layers, therefore the lock all/other layers is not applicable. There is no "lock all sublayers" command.
I'm not seeing an issue with selecting all on a symbol sublayer in CS5 either. Option/Alt-click the circle to the right of the sublayer name.
Perhaps you need to reset preferences?
How does one select everything on a particular layer? Select>All selects everything in the document.
Sublayers in the top level document can still execute the lock all other layers command.
You'd maybe want to read the documentation about the layer panel. You can use the layer panel to select everything on a single layer.