Twenty-Twenty-Two Child Theme Mega Menu

For the ‘Everything’ submenu, you can target it and then set the width to simply fit the content.

nav.wp-container-2 li.wp-block-navigation-item.has-child ul.wp-block-navigation__submenu-container {
    width: fit-content;

This worked for me on StackBlitz, but you may need greater specificity (MDN) to target the element in your project. Note that you can also flag it as important (MDN)

width: fit-content !important;

!important can be a useful tool, especially when dealing with WordPress, but it’s not best practice to start slapping it on everything, tempting as it may be. I would suggest some extra reading on specificity and the important flag.

For the last point, I tried to leave a comment to clarify, but I can’t do so until I have more reputation I guess. Do you want ‘The Seasons’, ‘Plants & Animals’, and ‘News & Events’ submenus all to be the same height and then have a visible border around it? In other words, plants & animals and new & events will have blank space under them in order to match the height of the seasons.