>Java is not "just" a system directory, it's a set of coordinated
commands, some of those shared between the jre and the jdk, with the set

varying slightly between Java versions, JDK provider, etc.

What you are (primarily) describing is the path export variable to the java binary which is located in a JRE/JDK install.

In Ubuntu and Arch Linux this is done via a "default" system link in /usr/lib/jvm that links to the configured system wide JRE/JDK located in the same directory(/usr/lib/jvm). Surely both you and the Fedora Java maintainers know this or have at least thought of it?

>Writing good alternative rules given all those can be quite challenging
and brain-damaging (alternative rules must upgrade gracefully to the
next package versions)

Again, just system link to the correct folder like every other distro. alternatives doesn't need to concern itself with specific content. It could be empty or copy/pasted as far as it *should* care. As long as it follows the java-* format you can get a list of all JRE/JDK installs regardless of the name and have "default" system link point to it.

>I'm sure the maintainers of the Java packages would be delighted to have
someone to review all the alternative rules and fix up all the warts
that crept in since Java 1.3 times (back in my day, once you got
alternatives to work for a new jdk, you tried very hard to forget the
experience). Certainly, all the standardizing on openjdk after SUN sank
should have simplified the mess a little.

This has nothing to do with how the folders are named. See:

https://www.digitalocean.com/community/tutorials/how-to-install-java-with-apt-on-ubuntu-18-04

under "managing java".