Unless your in laws are different than mine, my parents, and the parents of almost every other person I know who has gone through this, they will not cooperate. I think it's for a few reasons. 1) They are scared and in denial 2) as people age, they lose the cognitive ability to make good decisions (of course, this assumes it was ever there) 3) A desire to remain independent while simultaneously ignoring the huge burdens that places on the people who care about them, while not actually being independent but rather dependent.
2 things to get they probably won't want to give you: 1) A springing general power of attorney 2) Health care POA. Those 2 things make life much easier.
I see 3 potential courses of action. 1) Ask nicely, using logic and facts. It sounds like you've already done this. 2) Withdraw all help-no visits, no grandkids, etc. Let them make their own bed and lie in it until the day comes when you just show up and move them. Option 3) spring into action whenever they need it, but at the cost of your own life and family (this is what most people end up doing-me too, although I did try 1 and a version of 2). With the benefit of hindsight, I would have used the springing power of attorney and started World War III, and it would have been the right thing for everyone.
I would not find or provide any support now. That's just enabling poor choices. If you mil wants to, she can, but you should not. It just makes it harder, not easier.
Having gone through this, the best possible course is to move into a continuum of care retirement center, where there is independent living, assisted living, a memory care unit, and skilled nursing. It provides a community, and a safe place. It can be expensive, but depending on your location, may not be that much more than their current spending as all food, utilities, etc are covered. It also lets them age in place while remaining as independent as possible. I recommend looking at faith based CRCRs, regardless of your personal religious inclinations. They are usually the best ones as they prioritize resident welfare over profit.
I'd also add, memory issues get worse exponentially. There is no plateau, and the things you don't know are happening are even worse than what you do know. Memory loss is an ugly, ugly disease.
There is no good solution, unfortunately. You can only do the best you can. Whatever you do, don't let yourself feel guilty, for either what you do / don't do, or how your in laws react to your actions.