(I have absolutely no idea how this web interfaces quotes stuff, so you'll have to use some imagination

No. Commodities is a way to install custom input handlers into the input.device's even stream. Application.library is something different.
Commodities Exchange appears to offer some of the functionality offered through Amidock using application.library, with the infrastructure in place to do much more. At least, *I've* been using it in this way for years. I can't speak for others.
No. There is no standard on the Amiga. Some programs store their settings in S:, others in ENV:, and others in the application's
I believe that even with application.library it would be possible to save settings in S: (unless S: is replaced with /etc/rc/ or something). Just because people don't follow The Ways Things Are Done (TWTAD) doesn't mean that TWTAD isn't a defined standard.
Your highly-acclaimed commodities library
Do not put words into my mouth. I am perfectly capable of speaking for myself without anyone else voicing their opinions in my name.
*did* start out as a 3rd pary tool (on a fish-disk, I think somewhere around 50 or so, if you know what that is),
Don't be rude. It doesn't help to win arguments and influence people.
It's on disk 83.
and is now an OS component. Application.library is as much an OS component as e.g. datatypes.
I don't deny this and didn't suggest it would not be the case. Originally I only stated my disappointment that duplicated functionally appears to be included in the OS. I can't influence this at all, and god knows I'd rather let competent people do the hard work for me anyway.
So, I'd prefer an explanation of the reasoning behind it rather than what I've read so far!