@arvind_g3aero - how many cameras are you running? Can you paste the logs exactly - it is hard to understand the issue without the logs.
As for snapshot - when you change the stream in qgc, the snapshot is probably not tied to the new rtsp steam, causing a fault on the backend - I can look into this at somepoint in the next fwe days.
As for 1.3.5 and 1.4.0 those SDK's do NOT support multi camera streaming into qgc.
Zach