hello madraida,
I may have some answers to your inquiries. I hope it's not yet too late. heheh...
I have a pre-paid Sun sim card and was able to contact Sun's Hotline regarding International Roaming for Pre-paid users before I left the Philippines around 2 months ago.
Here are some of my answers to your questions =) :
1. The agent told me international roaming for Sun is not available for Pre-paid users, only for Post-paid subscribers.
2. I did not ask the agent about this because of the answer in number 1 heheh... but I think Post-paid subscribers are allowed with voice calls when roaming. I know someone whose friend often calls her using her Sun number from the States.
3. to 7. Sorry, I wasn't able to ask those things because of, well, their answer in number 1 hehehhe...
I know their website is not of so much help heheh... but I hope they would reply to you soon.