Is it necessary to repeat Istinja if impurity appears after some time of defecation due to muscle relaxation? And is Ghusl from Janabah sufficient if water does not reach the anal sphincter due to muscle contraction?

It is obligatory to wash the area until it is clean, while sufficiency is achieved by dominant conjecture, and by washing what appears from the exit point. What is inside the exit point and cannot be touched by hand is considered internal and is not required to be washed. If something emerges after ablution (Istinja’) and appears on the outer part of the exit point, which can be touched by hand, then it is obligatory to perform Istinja’ from it before performing the next prayer. This does not necessitate a ritual bath (ghusl janaba).