Great questions.
Initially, we are only on ETH but we are working on a polygon staking option too. We are also partnering with a bunch of different yield farming dapps on Polygon so that will come.

Yeah reward will be in sEURO to start with but eventually, we will have other options. especially when we start rolling out other sFIAT's like sUSD, sINR, sAUD, sGBP and so on.

Regarding people dumping rewards, yeah that could happen but it would happen after the staking period ends and we will roll them out in staggered tranches of new staking pools. 
There are also soooo many use cases and yield farming ops coming for TST so the smart hodlers should understand that.