Hi Keptray,
I can ans your 1st question, as I am still waiting for my visa grant.
1) as far as I know, health undertaking is for those with some sort of chronic illness (although some ppl don't even know why they were issued one! But i know my condition will require one)which require monitoring I.e. either blood test now and then or physical examination. They just want to keep an eye off you just in case there is a change in your condition. The health form will be sent by GH to your CO once they satisfy with your health requirement only if you agreed to be 'monitored' when you arrived in Australia. So from there on, CO will take over from GH, and they will issue you a visa once you sign the form and return to your CO.
Do you know whether you have a condition which might require extra considerations by GH plus if you apply for permenent visa, the health requirement is more stringent so it will take longer. I hope you will get there soon! Good luck