(2) Is it allowed to detain gas during salah and complete the salah?
بسم الله الرحمن الرحيم
(1) Yes, it is necessary to break the salah and perform wudhu. He should go back putting his hand on his nose and then join the salah in the same rukn in which wudhu was broken or offer salah again.
(2) It is makrooh tahrimi to stop it to an extent that one feels discomfort and the khushu and khuzu of salah is gone.
Allah knows Best!
Darul Uloom Deoband