Hi Muhammad Sami
1)I do not know why you can not change pressure, temperature.
Node 10 to 20 feed T1=60 deg, Node 20 to 30 you can change it to 21 deg,again node 30 to node 40 can change to 60 deg.

2)Yes you got rigid body concept correctly. Regarding SIF it is quite tricky one. You have to calculate it BY Finite element method.If your system is not so much critical may be you can use SIF of that elbow with some additional safety margin. B31.3 to not specify of the same.