Is it possible to recover an account i used to own from its new new "owners"

If i don't remove the former account, can i use my own live gold account if the live account from the former owner expirers? If this works i can buy and install new games on my own (my Sons) account and use the former account only for the already installed
games, maby in the future it is possible to switch the installed games to another account?

The hard drive uses Flash Memory so the data is possible to recover, i suggest sending the Xbox 360 to repairment and ask them to recover all data from the old HDD to a new HDD. You can register the Xbox 360 console with its serial code and send it to repairment
here: https://support.xbox.com/en-us/my-account/warranty-and-service/getting-your-product-serviced

Not possible transfer content between accounts, if he recover the account and it's the owner of the email he won't have issues

It's not possible to transfer ownership of an account. In fact, it's a violation of the code of conduct to give someone your account which would put the account at risk for being banned. It would be best to remove your account from the console and have
the new owner create a new profile for themselves.

What I recommend is to mark the customer service of xbox, there will solve that problem you will recover your account and a new password, as long as you prove that you are the owner