Sorry to hear about your frustration with the same issue Jefferson. I really don't think playing the slideshow has anything to do with this issue. My efforts to clear my website data and play the slideshow have been fruitless as well. We now know two ways the problem will not get fixed. Maybe we can define the problem better for those who might have other solutions.
I did the El Capitan update and my extensions were listed in my toolbar as you would expect.
For the first time since I began using a Mac (around 1991), I was infected with adware. The usual crap. It snuck through after I updated to El Capitan. My ad blocker software wasn't up to date yet. To get rid of the adware I used and app called Adware Cleaner. Did the job but I suspect it also erased the files that connected Safari to my User Account. (Preference files? .plist problem?) You may be experiencing the same issue. I don't know if user preferences work the same way in El Capitan. There has to be a way to re-connect our User Accounts to Safari again to allow us to access the 'Install Now' button state. I, too, do not want to do a clean install of Safari. Everything else works fine but I sure miss my extensions.
Does anyone know how the relationship between a User Account and the Safari extensions page works?