This is useful in my case when syncing tracks to a mobile device. My main library has about 25k songs across 2k albums. But based on device capacity I tend to sync about 2-3k tracks at a time and change them up regularly. Having the artwork embedded ensures these are transferred without having to configure the sync to somehow always grab the cover art from each folder.
I have never noticed a significant difference when scanning the library, whether on my main library PC or any mobile device.
This is correct. And since the text fields have very little impact on file size but carry the most weight for maintaining a library it is essential for them to exist. Reducing the artwork file size to allow these to exist is most important.
This is the most important thing. Do what works for you and your requirements. The best advice is to be consistent, regardless of how you prefer to manage your music.