What is the difference between "Aku" and "Saya"?
'Saya' is formal, 'aku' is informal.
Could bakan have been used instead of tidak?
No. Bukan is the negative form of a noun, for example "Saya bukan guru" - "I am not a teacher", whereas tidak is the negative form of verbs, adjectives and prepositions
Here is more info:
Negation: Tidak, bukan, jangan, belum, Tips & Notes, Addendum.
Kek and kue is the same?