Supercharge.info lists the opening date as 2014-10-17.
On October 14, user Hiperdino shares construction pictures.
On October 18, user lasa reports everything looking ready, nobody has tested yet.
Later on October 18, user patrickCH reports a successful charge.
On November 1 there was a double opening ceremony for Maienfeld+Beckenried, with pictures shared by elekktrisch.
All construction and opening pictures show just 4 stalls. The future extension was probably taken into account as chargers were numbered from right to left, so that stalls 3A and 3B added later would be in sequence.
On March 1st, 2017, user NCC-1701 shared that Beckenried has 2 additional charging spots from that day. No pictures. On March 12 we see the first picture of the upgraded station, but it's apparently been re-posted from Chargemap.