Looking more closely at your errors, it appears that your package cache may be corrupt. When the installer attempts to read from the cache, it runs into a JSON error. As a workaround, you can try removing the packages from %ProgramData%\Microsoft\VisualStudio\Packages (All folders EXCEPT _Instances). That might unblock you.
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2017 RC won't install, says "Starting download/install forever"
This issue is read only, because it has been in Closed–Fixed state for over 90 days. It was closed for 777 days.
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This issue has been fixed. You can download the latest version of Visual Studio via the in-product notification or from here:https://www.visualstudio.com. Thanks to everyone who provided feedback, votes and solutions for this problem.
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I have installed the fix by upgrading to the latest version of Visual Studio. The issue is still not fixed: