What is the criterion for incapacity? And does an obligation become nullified with incapacity, making a concession obligatory, such as gesturing in prayer for one able to stand, or reciting silently, or performing what one is able to perform of a deed? And does the hadith apply to it: "If I command you to do something, then do of it what you are able to do"?

The jurisprudential rule "No obligation with inability" is sound and established by evidence. The criterion for inability is either complete lack of capacity or the presence of significant hardship. What is difficult but not harmful does not cease to be an obligation; rather, the Lawgiver encouraged enduring it. Taking advantage of a concession (rukhsa) is not obligatory except for one who is unable to fulfill the determined command ('azīma). Scholars differed on whether the 'azīma or the rukhsa is preferable for one who is capable, and some differentiated based on the مكلف (accountable person)'s situation and ability. Taking the legal concession is better for the مكلف, based on the Prophet's (peace be upon him) saying: "Indeed, Allah, the Blessed, the Most High, loves that His concessions be availed of, just as He dislikes that His disobedience be committed," and his saying: "Indeed, Allah loves that His concessions be availed of, just as He loves that His determined commands be fulfilled." As for taking the easier opinion from scholars' fatwas that aligns with one's whims, this is not permissible. What is obligatory is to be cautious in matters of religion and to follow the evidence.