2. This high interest rate will only last for 9 months... so might as well take advantage of it
What makes you think that? I don't think that is correct. The Steem Power compounding is perpetual.
I read form a post by dan himself if I remember correctly... I think the interest rate changes
I am bit confused about this. There is one form of compounding that comes from the perpetual creation of 9X Steem Power for every 1 Steem that is minted. Those 9X SP are distributed proportionally to all SP holders. So your SP holdings are always increasing. I don't think it ever ceases nor changes.
Is there also another form of interest paid on SP?
The 9-1 ratio between SP and Steem never changes. I'm pretty sure what he means by the "high interest rate" is the numerical reduction in the rate of effective interest as the supply increases.
Example starting with a supply of one coin:
Add first coin (interest/inflation is 100%)
Add second coin (interest/inflation is 50%)
Add fourth coin (interest/inflation is 33 1/3%)
etc.