The Crown sale ended on April 13, IIRC. The scrolling text at the bottom of the launcher screen mentioned the Crown sale, the Anniversary event, etc. and it stated that the Crown sale would end on Aoril 13 (or whatever the date was), so I don't think it ended early on Steam. I made sure to check for the date it would end so I could be sure to buy some before the sale ended. But the sale on the game is still on (I think), although it ends tomorrow-- April 20. I'm sorry you missed out; that's happened to me before, too, so whenever I hear they're on sale I check to see if the announcement says when it ends.
You only need to buy the chapters through Steam you can buy crowns directly from the ESO website, I have the Steam version never once bought crowns from Steam.
I bought ESO through Steam, launch it from Steam, and buy Crown packs and Chapter expansions in the Steam Store. As far as I know, the prices in the Steam Store always reflect the current prices from ZOS, including sale prices and the dates when specific sales events are in effect. As I mentioned in my previous response, I specifically checked the information given in the scrolling text at the bottom of the ESO launcher window-- or I guess it's the server-selection screen that has the scrolling text announcements, not the launcher window-- and that text did say that the sale on Crown packs would be until April 13, despite the fact that the game is on sale until April 20.