Its a easy way of earning btc and increasing your fund .Most of the platform are giving almost the same APR rate .You can get interest and can earn .I think you can get a good idea that he who invested and get interest in the past on btc just calculate now how much they have gotten the return .We need to think positively if we always think the market could fall we can't find a good return even after getting the interest .I think investing for interest is long term invested if you really wanna see a good result on that .You were saying he who own the privet key own the wallet ,its true so why you need to share the key when it is really need to store securely .