Hi M
You monitoring internet usage etc would make no difference to him gambling, There are things I believe that can be installed  to block these sites and create barriers but unless someone wants to stop gambling M they will always find a way to do it.
Without sitting in judgement of your FIL I believe what he is saying to you is wrong on many levels. It is wrong for your hubby for him to suggest that control of his addiction is down to you and that the results are in some ways are your fault, this only serves to help your hubby take less responsibility because this is what addiction wants to hear.
It is wrong and harmfull to you as well for someone to try and give such a responsibility to you and leave you in such a position where it is possible for you to feel guilty and feel that you should do more or should have done this that and the other, it sounds like he is in denial but is bullying you with his nonsense.
Even if your finances had been better protected at the time – he would still have gambled, enablement is giving someone the means to gamble and if he took it from you without your knowledge then that is not enablement on your part it is theft on his part and abusive of the trust you showed him. Prevention of this occuring again is all you can do to protect yourself.
If FIL thinks that assisting him to get loans is helpfull then good luck to him but I have never heard of that ‘helping someone with a gambling addiction’
I understand now what you meant by control and I see your point but from the point of view that his dads view is also demoralising you as a person and creating doubt in your mind. You are not responsible, but you are strong and you are doing well. Put your efforts in to you by dismissing from your thoughts the ramblings of this man and know that you are right in your own beliefs. Looking after you means you look to you to make you happy, it doesn’t depend on the actions of others ***