My ex partner is a CG alledgedly now in recent recovery. I feel that your father in law will soon learn a sour lesson with regards to enablement and manipulation and I would have thought the least he would have done is appreciate the fact that his son was rescued from the sea and not pushed right into it !!!! However it appears that you may be dealing with 2 people with gambling issues not one, both of whom have their own agenda, perhaps achnowledging his sons issue properly may lead him to look at himself. Someone once said to me that which you dislike in other people may be what you fear in yourself.
I would not use the word control in relaition to your CG too much if I were you, you will never control him and neither will your FIL and if you feel that you are in control of him at the moment then that is not really a good sign. Being in control of finances and in control of yourself is different from controlling him. The only person that can address his gambling is him.
It is good that you plan to put your children first and unlikely that they can be totally protected from what is going on but you need to put you first to. Your children will be happier if you are happier!!
I think many of us have developed our private detective skills to the point where they are second to none and bordering on being obssesive, much of our day taken up by checking up on our partners activity or in wondering what they are doing, when does the next bombshell land etc etc. It is possible that we can put nearly as much effort into gambling as they do!! unfortunately this is a game we can never stay ahead of but burn ourselves out trying. If a CG wants to gamble he will do so and so the lies and deceit get bigger. It is a waste of time and it is time taken from our children and our own lives become empty. Next time you feel the urge to check up, try doing something nice instead.
Where money is concerned never think you know for fact what maybe the truth, it is highly likely your FIL has been giving him money but that your partner is better at concealing this fact than you are at discovering it. Other people who enable eventually find out they have made a mistake and infact have only helped him to become sicker by feeding his addiction. What is important is that your own finances are now protected.
You are not responsible for his addiction. He is the only one that is responsible for his actions and also his own recovery, you can support him but ultimately he has to do it for himself. The only person you can change is you xx