1- Yes it us valid. Islamically it doesn’t have to be written. A verbal agreement is sufficient. However, know that this is not a binding contract. It’s a promise from him to give you that profit. And he should keep his promise, but if he doesn’t he is not violating a contract.
2- Based on the promise he made, yes ethically he should give you those profits forever.
3- If there was not agreement that you would take losses, then you don’t have to take any losses.
4- It makes it justified as long as he willingly agreed to it and promised it to you.