As a little disclaimer, I'm going to get the terminology wrong in this question, but please bear with me.
There's plenty of questions and answers I've looked at in my journey of troubleshooting so far trying to merge shapes in My Maps.
I've been able to create polygons and merge them by right clicking, but I have other shapes, styleURL #icon-1739-FFD600 from the KML file, that aren't able to work the same magic. I did a find-and-replace to change the styleURL from #icon-1739-FFD600 to #poly-000000-1200-77-nodesc, but had no luck. It did change how things looked on maps, but right clicking outside two adjacent shapes' bounds only gave me the option to delete selected.
I've also tried to simply take the folder references out of the KML file as another answer suggested, but the ends of the shapes didn't line up resulting in choppy, unexpected areas:
If there would be any way to put these in the right order before the folder references are cut out of the file, I'm sure that would solve the issue, also.
All I need is the U.S. States as polygons in a format where I can merge them.
These are only the routes I have tried to go in effort of avoiding the worst-case scenario, drawing them all by hand with accuracy.