I couldn't say you were wrong, but here "由" and "于" are quite different in meaning from the phrase "由于". In the phrase "由于", which can mean 'because', 'as', 'since' or 'for', etc., "由" and "于" cannot be separated. In the given sentence "由他于祭典中...", "由" means 'by...', like 'sth. be done by sb.', actually '由' has lots of meanings, and "于" means 'in...', which can also mean 'among...', 'during...' or 'at...' in other situations. The incompleted sentence you gave above might mean 'sth. has been done by him in the ceremony (or fiesta).'