Will Agate coin be minable when main net is launched?
No, that is POW and Agate is POS.That means you will be able to stake Agate instead of mining it.
Will there be some minimum amount needed to join stake campaign?
To start masternode you will need atleast 50,000 AGT.But if you have less you will be able to join public pools.
Pretty large amounts, how much does it in dollars/btc? If the project earns what roi can you expect? If above average, then the project clearly deserves the attention of those who like pos/masternodes.
It will be profitable, especially in the begining as it is with every masternode, but hard to say long term as ROI is impossible to calculate at this moment.