Bit of a newbie to SAA question:

I am from the US but I live and work in Malawi and travel back and forth frequently. I've always credited my JNB - JFK flights to UA because that's my preferred mileage to earn but because it doesn't get me any status I've been debating switching to SAA. I have three questions:

1) I have Star gold on Aegean, would SAA status match me or let me do a platinum challenge since I'll be flying about 50k miles on SAA over the next 6 months or do I have to push to try to make 100k before Dec 31 this year?

2) Do you get upgraded on long haul international flights like JNB - JFK when booked into the cheapest economy seat if you're platinum. What is the upgrade cost using miles that I read about?

3) Is it easy and advisable to redeem SAA miles for award flights that it would be worth switching from United which I've found very easy to SAA? The partner award chart seems to suggest it's more expensive for most award tickets but are there other perks to the SAA program that im missing?

Any help with this is most appreciated.