Yell and shout should both be acceptable.
report if it's not
Is "kiabálni" here composed of "ki+abálni" or is it a verb without particle?
The latter. There is a rare and probably unrelated abálni verb, though.
Yell and shout mean the same thing
So with multiple vowels in a word which one is used to determine the suffix of the verb?
Typically the last one. With i, it's common that preceding back vowels override it, with é, it's like both can be accepted depending on the speaker.