"Kein" is used to negate nouns and "nicht" is used to negate adjectives, adverbs and verbs. I'm just a beginner but I know most of the basics, I hope this helps :)

"keinen" is for masculine accusative, and while this is accusative, Brot is actually neuter (das Brot), so it remains as "kein".

The rule is that Kein is used when the sentence has 'a/an' or when the sentence has no article(a/an/the) where as nicht is used with definite article(the). Here the sentence does not have any article hence we would use kein.

No, that's simply wrong.