Terminal command to list application defaults?
Not easily. You can use the well-hidden lsregister command to dump the contents of the Launch Services database, which is what controls which file types open with which applications:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -dump


but its output isn't well documented -- there's some info here, but it's over 10 years old so it's probably at least partially obsolete. A bit of Googling turns up some AppleScripts for further parsing, that you might be able to adopt:


https://stackoverflow.com/a/3452435/476909
https://macosx.com/threads/howto-list-your-installed-applications.26204/