Here comes a native speaker. First, ばか/バカ/馬鹿 is not slang nor swear. It’s just a word like stupid, or idiot. Second, there’s no rules nor solid customs to decide the choice, but each gives off subtly different impressions: ばか: somewhat aggressive, imo バカ: somewhat humorous, imo 馬鹿: somewhat obsolete, imp ばか野郎/バカ野郎/馬鹿野郎 is just a compound word. He’d lose the argument. *Note that Japanese doesn’t have so many slang like English, which may confuse learners, because many of slang such as ‘dumbass’ happens to be translated into バカ.

as it's a native Japanese word -- it should be written with hiragana

Sometimes you write Hiragana words in Katakana. The reason is based more in the emphasis you want to give to the expression. For example, in a manga, an insult as "baka" is suppose to be said with emphasis because you may be angry, upset, etc. It's not a proper rule, so I can't be absolutely sure, but sometimes works as ,when writing in English for example, we use capital letters to give to the word more visual power.

oops; not sure if that kanji is right, as it doesn't match my dictionary for the word. ばかやろう is what I was trying to write for the elongated version. Thanks!