Hi,
what you describe from his behaviour is exactly what a friend of mine told me about her abusive husband, that she's now trying to leave. He moves around her stuff every now and then, especially the things she likes and wears often, just to drive her crazy and make her self-doubt. She knows it's him but he obviously denies it. This is certainly not dementia! If it were dementia, he would be losing his own stuff and forget his plans for the day, forget things he was saying, get lost etc. He wouldn't be so sure of himself. My grandmother had dementia and it's nothing like what you describe. He's targeting your stuff deliberately, it's a strategy of violence. It's used to destabilise and weaken your own judgement. If you no longer trust your perceptions, then it increases your tolerance to violence, as you're more likely to think you imagined it, that it's in your head, or that it's your fault.
The other thing you say about him is quite alarming, that he moans about you to the children for things he hasn't done. He is aware of what he's doing, and the deliberate effect of this is to turn your children against you. Again, this is a very typical strategy of abuse: abusers recruit allies, devalorise the victim in the eyes of others to isolate her, make others think she's crazy, stupid...
And you also say he swears in front of you: even if it's not directly against you, how does it make you feel? If someone were unpredictable and for very small things like that in front of me it would certainly make me hypervigilant and anxious not to make him angry. And unpredictability, sudden moodswings from nice to angry for very minor things is also abusive.
All this seems very serious to me and staying with him is likely to affect your physical and mental health badly, especially on the long run.
You say that he also has good sides and you'd like to save this: of course, it's important to aknowledge them. Most abusers are human and you wouldn't have gone into a relationship with him if he didn't have good sides: if he were only violent and never nice, you would have left immediately. But the thing with abusers is that the good sides are what keeps us attached to him in spite of his violence. It makes us forget the violence, or want to forget it, or blame ourselves for it. We question the reality of it. We want to believe that only the nice side is real and that his violence isn't real. And this in itself is very dangerous because it's what can keep us 20 years or more in a very abusive relationship because it fuels our hope that he will change, that one day he will stop being violent. So actually his being nice and caring every now and then is part of the danger. It makes him more dangerous because it destroys our capacity to identify his violence and protect ourselves from him by cutting all contact.
abusers don't ever stop being abusive to their victims. The abuse doesn't change, what changes are their strategies. If they notice we start resisting more, or that their strategies don't work on us as much as they used to, or if they notice we're planning to leave, they can make a U-turn and change their behaviour for a while just long enough to deceive us.
You're not responsible for this. It's difficult to accept that the one we love is the one who is dangerous and destructive to us, but it's his fault, not yours. He's responsible for it. You need to protect yourself and leave as soon as you feel ready to do so. He is very dangerous to your physical and mental health.