No, you can use longer URLs, but try to keep them short
Google might pick a shorter URL over longer one in certain cases
Are URLs case sensitive ? Yes, search engines can understand the difference
use rel = canonical where applicable
rel = canonical
I don’t understand SEO but for humans yes 100% unless the user decides to bookmark the site.
It depends on the name too. I made my forum URL to be 2 words that can be easy to remember. If it's too short, they don't even form words anymore. But since most people bookmark websites, it doesn't really matter the length. Now from SEO's standpoint, I can see that it may be indexed better.
Here URL is used in the sense
I would say, within reason, shorter URLs are better. I guess it depends on how you want to brand your site, though. For some sites, a slightly longer URL may be warranted.