Hello Anais.C
Regarding the first issue (impossible to define SpaceClaim as default program for the files .step, .stl,...) i will check with the development team and let you know.
Regarding the second issue (when I double click on a SCDOC file in the Windows browser and SpaceClaim is already opened, the file is opened in a new session SpaceClaim and not in a new tab) I checked in both R19.1 and R19.2 version of Discovery SpaceClaim and in both versions, the file opens in a new session. In which version you noticed that the file gets opened in existing SpaceClaim session?
You are navigating away from the AIS Discovery experience