question concerning diana's aunt....spoiler...

Interesting question -- maybe it was protective instinct (although unnecessary) or perhaps she didn't truly know the extent of Diana's divine nature?

I suppose you can theorise that the Amazons wouldn't know specifically what could hurt/kill Diana, or to what extent she is a god, so why risk it?

Ares is a full god, Diana is only half-god, half-Amazon. Also, he may be referring to a fully-trained god. A lot of people tried to kill WW in the movie, but they didn't succeed, not because she's invulnerable, but because she developed enough skill and training to back it up.

If you remember back, there was shown a continous evolution of Diana and her powers. Hence why she needed the harsh training to tap into her potential and start her "ascension".

At the very beginning of the movie, she was basically a strong amazon in both mind and power for all intents and purposes.

At the end, she was a fully fledged demigod - almost invulnerable, strong af and some of the godlike powers.

Which is why she had a scratch on her arm after the first altercation with the germans on the beach (which healed very fast) - scratches she wouldn't get in the end, or BvS. ;)

You are assuming Antiope has all the knowledge that Ares has. And that Diana has reached full Godhood already. And that Antiope knows that the German guns are not weapons of Ares, whom she has been expecting to invade her whole life. etc

all of these thoughts make sense.

In a movie, a ninja might say only a ninja can kill another ninja. Perfectly logical statement, but it's because what that ninja has learned combined with his natural talents has given him great defensive ability. It's not like a statement about physics or natural law.