Tried that, didn't change anything.
Here are the results from the element tab:
Interesting, the install button doesn’t appear to be blocked by anything, it just isn’t there. And you’re using Safari 9 on El Capitan? And safari-extensions.apple.com opens when you click on “Safari > Safari Extensions…” in the menu bar (instead of extensions.apple.com)?
Edit: I also noticed that in one the screenshots above, this line appears:
I don’t see that anywhere when I view that page.
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