E has several different meanings or uses, depending on context. In the case of Aloha e Kaleo, "e" acts as an object marker. Youʻre saying Aloha (to) Kaleo. When starting a sentence, it often indicates the start of a command, like: E Kaleo, e pani i ke kula (Hey Kaleo, close the door!)
ABOUT THE LACK OF AUDIO EXAMPLES: No, its neither a lag on your phone, nor the app Its just that the Hawaiian language is in fact indangered, thus not many people are fluent speakers. That is why the Duolingo app has few sound examples; because they havent veen developed due to the lack if "staff".
I think maybe you're getting bogged down on the wrong thing. It might be true that the app accepted "greetings" but not "hello" or vice versa. That should be fixed, so click the REPORT button.
The bigger question though is what does a Hawaiian word "mean". Hawaiian is not just a secret coded way to speak English. Words don't have a 1-to-1 correspondence between any two languages. This is masked from us because, in languages like Spanish or Italian, most of the time you can pretty much find such a 1-to-1 correspondence.
But Hawaiian is different. In fact, "Aloha e Kaleo" strictly means neither "hello" nor "greetings." ... "aloha" is a stative verb that expresses "love". So the sentence means literally something like "I am expressing the state of sending love to Kaleo." We wouldn't translate this literally, of course; so "hello, Kaleo" or "greetings, Kaleo" are both reasonable translations of the overall meaning of the sentence.