Do your research, old versions mean you can’t simply take the latest version of everything, you need to find the specific versions that work. If you insist on using old versions you can’t always expect support.
Because Forge's MDK ships with a very old version of Gradle, one that was not designed to be compatible with Java 9+ AFAIK.
If you need to keep Java 10 on your computer I believe Gradle will use whatever Java installation that the JAVA_HOME environment variable points to.
Trying to open the installer in explorer? No no no, you open the folder that has the installer in it in explorer, not the installer itself. Usually this will be your downloads folder.
If it's on your own computer just use localhost as the server address.
Unless friends are joining you won't need Hamachi, and if you do have friends joining they will need your Hamachi address, you won't use it yourself.