1.Yes, your husband's signature is required for getting Visa for your son . You can not take your son out of the country without his knowledge and consent and he can certainly pray for a restrarin order on you from the court for the said act of yours,
2. You have married in India and he is staying in India. So, you can not file a divorce suit in Australia and avail a decree of divorce in the given circumstances. You shall have to file divorce suit in India by yourself and your POA holder can pursue the matter further in the Court. You can file the divorce suit on the ground of cruelty. GPA can be executed in favour of anybody,
3. In both the case you shall have to personally be present in India while filing the case. MCD will be disposed of with in 6 & 1/2 months and you shall have to personally appear before the Court on the day of the 2nd motion when the said MCD will be decreed of,
4. Yes, it is a valid ground for seeking custody of your child for his welfare.