As you already guessed, you can't trust any server or client you do not have under your control. So you can't send processed results. You have to send requests to the server, indicating the actions that lead up to these results, and verify these requests against the master server's knowledge of the rules of your particular game.
Even then, someone could hack the client and e.g. create aim-bots or whatever to make the game easier for them.
If your users run the servers, you can guarantee even less. The person hosting the server (or "master client" or whatever) could be running a hacked server, you can't know. So if that's your approach, you have to decide how much you want to trust them.
What is your motivation behind this P2P approach?
If the objective is that friends can play and have fun without having to do it online with others, or in more privacy, then it's probably fine, because a friend who runs a hacked server will get reprimanded by his friends. You just can't take any results from these off-line games and use them on your official server.
If the objective is to make it easy to find other players nearby for multi-player matches, you can just make the finding of the partners P2P, and then have each player send the unique ID of their team to the central server. You get easy discovery of P2P, and less cheatable gameplay of a central server.
If the objective is to save yourself from having to run a huge server, you could try to split up the operations across all users that are logged in. I.e. send out little units of calculation, maybe even unrelated to the game of the player whose machine is processing them. That way, nobody can predict who will benefit from any hacks they do, and if you have several computers perform the same calculation, you can find hacked servers because their results don't match.
Or you could simply forget about it, like many single-player games on iOS do. They simply let the user play the game, and then send the result to a high score server. Anyone can send their own fake result there (in fact, many iOS games these days have the first bunch of top high score spots taken up with ridiculously huge hacked numbers). But since the high score is mainly an encouragement to users and doesn't really affect gameplay or rewards you might get, it's OK in that case.