Are other items, such as staves and clubs in the scope of next update, or will that be later release (I can see this ballooning very quick)
Other tools wear and tear additions will remain for later releases. It's probably not worthwhile to add these mechanics for all the utensils you may need in
crafting and building tasks, but for the relevant ones.
However, as we know all the weapons are already prone to damage in combat situations.
Can we expect wear and tear to be moddable; now/later/not?
I'm not sure what type of modding you are thinking of there, but the answer is yes.
Axes and shovels are automatically included in the wear out mechanics in all the crafting and building task - including diy_*.txt and biy_*.txt files.
The level of wear out can be specified with modding tags since in some tasks the tool may be used less intensely than in others.
You can also add tag to completely exclude the wear out mechanics for some entries, if need be.