You have asked quite a few questions.
Finished movies, by which I assume you mean movies shared to the Theater, will display in the Theater browser screen in the order in which they were added, so that the last added will appear on top. They cannot be organized in an other way. They can be renamed by selecting the movie's icon in the Theater browser screen, clicking on the little circle to the right of the title, and selecting to rename it in the drop down menu that appears. You can type the year into the title if you want.
If you are talking about finished projects, you can sort them by name, date, or duration by ascending or descending order. Go to the projects browser screen and click on View/Sort By in the iMovie menu.
To rename projects you select it and click on the little circle next to the name and select to rename it in the
drop down menu. Again, you can type the year into the title if you want.
You are correct that there is no feature to allow selecting the thumbnail (or poster) of either projects or movies shared to theater.
As to being unable to use songs from Apple Music, that possibly could be because they are DRM protected (Digital Management Rights).
As for Apple's decisions in designing its software, you would need to ask Apple about that. This forum is comprised of users like yourself who have no control over what Apple does.
Here is a link to a help menu with a good search feature that will describe how to use the functions of iMovie:
To view and manage projects:
To organize media and events:
To locate source files for your clips: