Is it obligatory to delay ablution for Friday prayer until the sermon begins, in order to ensure the time has entered, and is delaying the Friday prayer in this case considered a sin, or is one excused?

There is no harm in performing ablution (Wudu') for Jumu'ah (Friday prayer) after the Khatib (preacher) has ascended the minbar, and there is no sin in that. There is no harm for a person with سلس (incontinence/continuous discharge) to perform ablution before Zawwal (declination of the sun from its zenith); acting upon the opinion of those who hold that the time for Jumu'ah begins before Zawwal. By doing so, one can arrive early for Jumu'ah prayer and attain the reward.