Staking is basically delegating your coins/tokens to support the chain in performing POF (Proof of stake), that way you are supporting transaction validation and this improve the security of that chain.

I will answer your questions
1. To reduce inflation and spamming of the network, some measures are always apply when it come to staking.
For example if there is an annual inflation of 3% of total supply been dedicated to staking, that will increase the supply in time, they have to make sure it doesn't affect the price so, when you see 10% for staking, it will vary as more and more people stake or unstake.
Let's say token A has 20 million for stakers at 10%, the percentage will decrease as more and more people stake and vice versa.

2. All tokens or coins are good for staking, depends on your choice of projects. Low Cap project will promise you heaven and see hell at the end, choose wisely.

3. I'm not sure about that your value but at that value ($214) and the current price, this is the amount you will get in total.



This is the yield you will have after a year provided that it's stable at this rate, if it varies, yiur yield might increase or decrease