To open a file with a double click is not possible in Dolphin 15.12.3. I cannot find a setting to select a file with the one click and open it with a double click.
KDE System Settings > Input Devices > Mouse... More - KDE Forums: https://forum.kde.org/viewtopic.php?f=223&t=128669
*** This bug has been marked as a duplicate of bug 353645 ***