Is delivering a sermon on behalf of a designated preacher, who has no valid excuse, considered a form of cooperation in righteousness and da'wah (inviting to Islam), or is it assisting him in being negligent in his work for which he is paid?

If you are qualified for the religious position, there is no objection to asking the preacher for permission to deliver the Friday sermon. This is a form of cooperation in enjoining good and forbidding evil, and in conveying the religion, as the Prophet, peace and blessings be upon him, said: "Convey from me, even if it is a single verse," and "May Allah illuminate the face of a person who hears my saying, comprehends it, memorizes it, and conveys it."