On Safari Extension Gallery page the "Install Now" button does not appear after updated to Sierra 10.12.4. It seems that Apple's official webpage didn't recognize that I'm using Safari 10.1 (12603.1.30.0.34).
I searched the community and found lots of discussions of similar issue, but it seems there is no magic cure for everyone. Here is what I've done so far:
I can still install and activate 3rd party Extensions directly from .safariextz file. So the "Verisign Certificate" thing is not the cause of this problem. And that "Trust" button works as expected with normal mouse click (no need to enable Keyboard navigation or something).
Antivirus / CleanMyMac / MacKeeper / other 3rd. party in system proxy or firewall: NOT INSTALLED in the first place.
Log in with Guest account: NOT WORKING. Safari Extension Gallery still won't recognize me.
Reboot into Safe mode: NOT WORKING.
Fix permission with Onyx: NOT WORKING. It doesn't seem link to any files under user home directory anyway, since the Guest account also suffer from the same problem.
Delete "Safari Extension List" in Keychain Access: NOT WORKING.
Remove the whole "~/Library/Safari", "~/Library/Caches" and other Safari/Web*** related .plist file in SyncedPreferences and other places: NOT WORKING.
EtreCheck: Diagnostic result is pasted here: https://pastebin.com/zmUcLs2R
-- Thanks for Any Help!