Is it permissible not to make up missed prayers due to ignorance of the Sharʿī ruling, based on the opinion of Ibn Taymiyyah that the Prophet, peace and blessings be upon him, did not command Umar to repeat the prayer in a specific incident, and is this considered following concessions, with the fear of increasing Waswās?

There is no sin in acting upon the statement of Sheikh Al-Islam regarding the non-obligation of making up prayers for one who suffers from obsessive-compulsive disorder (waswasah). This is not considered blameworthy pursuit of concessions (rukhas), as the person afflicted with waswasah may adopt the easiest of opinions to alleviate hardship. You are not obliged to make up prayers whose obligation you are not certain of; for doubt concerning an act of worship after completing it is not to be heeded.