Error 1: Changing «أَحَدَ» to «أَحَدُ» solely because the compound is the subject. Correction: the fixed numeral is in the nominative position.
Error 2: Calling both elements of «اثْنَا عَشَرَ» equally indeclinable. Correction: the first declines as a dual.
Error 3: Calling the fatḥah in «مَرَرْتُ بِبَعْلَبَكَّ» accusative. Correction: it is genitive marking on the diptote-like second component.
Identify the governor, position and actual sign separately rather than memorizing labels without analysis.