The issue is you're updating your own Java, not the one the launcher is using.
When Twitch opens the vanilla launcher change the profile it gives you to use the Java you installed
There will no longer be a Windows-Specific installer, just make sure you have Java installed and use the regular one.
If you have issues please provide your installer log, if there's no installer log, run the jar manually from the command line and provide the output.
Forge installs itself as a version of Minecraft.
Make a new Profile/Installation and select Forge from the version dropdown.
(Note: For some reason Forge 1.12.2 gets booted to the bottom of the list.)
Do you not know how to find a file on your computer?
Do you not know how to copy and paste text?
Do you not know how to click a link showing where the file I'm asking for is on your computer?