The two endings are not contradictory:
Zaydan is accusative by the exception analysis.
Zaydun is nominative by the substitution analysis.
Do not ask only whether illā appears. Ask whether the clause is non-affirmative, whether the base is mentioned, and which allowed syntactic analysis is being used.
Opening interactive controls...
Al-Mustathnā: Connected and Disconnected Exceptions, Four Case Rules
The two endings are not contradictory:
Zaydan is accusative by the exception analysis.
Zaydun is nominative by the substitution analysis.
Do not ask only whether illā appears. Ask whether the clause is non-affirmative, whether the base is mentioned, and which allowed syntactic analysis is being used.