You cannot buy a Swiss Travel Pass more than 6 months in advance of the travel start date, which will be pre-validated. Special offers like extra days could come any time during the year, but more likely to be early than in peak travel season.
The Muerren-Schilthorn leg was fully covered in 2018, but we were told that a rival rail route started a lawsuit about that, which is why this route was for a time shown as discounted 50% on the Schilthorn's 2019 price list. The link Sam provided has been edited (since I printed a copy last month) to again say the whole route is covered, so the Schilthorn must have won their court case to be allowed to make that offer.
While the Swiss typically announce any product changes much further ahead than other railways in Europe, there can be reasons for features to change on shorter notice.
Tim, yes, it does mean that it's covered 100%, unless you're talking about an optional reserved boarding time. However, the nearby Mürren – Allmendhubel route is not fully covered.