I need help urgently!! Child custody, divorce, different nationalities

I need help urgently! I've been living in fear all these years.

We got married in Australia and now living in Singapore. He and my 4 yrs old child are Australian citizen and I am Singaporean. Can we file for divorce in Singapore? Or Australia? Which benefit me and my child more? As in term of child custody.

Can I use email as evidence for adultery in divorce? (which he doesn't know that I've already found out that he's been cheating on me). Besides that, he's a violent person, which he ever did to me whenever I tried to confront him. He has ever been very abusive to my child a few time whenever he's angry.

I am a full time mum staying home looking after my child since birth. Without a job, will that be disadvantage in fighting for full custody? I know joint custody will benefit my child but I'm worry that he might not return my child if I continue letting them seeing each other.

IMPORTANT: Will it be possible to file for divorce while I am away with my child? (overseas, where he can't get hold of us) We can't live with my family, it'll only cause more problems for them. Reason why I can't solve this matter with him face to face is because he'll definitely hurt me and my child, or use my child to threaten me if I don't leave the house alone.

PLEASE HELP!