Aggression is actually very normal. I used to be a caregiver in a memory care facility and it was unfortunately often the case that many residents were placed there by their family because they became physically combative towards their loved ones. It's sort of like becoming a toddler again, when they become unable to communicate their needs with words they will do whatever they feel they need to do to convey that they feel upset. The most compassionate way to handle that behavior is to remember that however irrational it seems to us, it makes perfect sense to do those things in the person's own mind.
Accusing others of stealing stuff is also very common. She probably feels very upset that she continuously can't find the things she needs and doesn't understand why this is happening. When she accuses you of stealing, reassure her that you'll take care of whatever is bothering her and if you can, offer to help her look for it.
People with dementia are often soothed by a reassuring voice, a warm hand on their shoulder, and just telling them whatever they're worrying about will be taken care of. Even when their verbal understanding skills are nearly gone, what you say doesn't matter as much as emoting sympathy with your tone of voice and body language.
If you can't help her look for the object or know it's something she doesn't have anymore, you can try to redirect her with going for a walk together or a favorite food or drink. Food is a very powerful motivator she may forget about her worries when her mind is focusing on enjoying a snack.
When it comes to telling her about her mom, I think it depends on the context. It would be cruel to tell her every time she says she wants to see her mom that her mom is dead, she would be starting the grieving process over every time. I usually say things like "Your mom loves you very much and she knows you're here. You'll see her again some day." (Not a lie because they'll meet in heaven… ) If she's still unconsolable after reassuring her and is becoming increasingly agitated, then it may be time to tell her the truth that her mom passed away and comfort her while she grieves. Use your best judgment about when that is appropriate.
When you're caring for someone with dementia, it's sometimes better to step into their reality. Does she need a shower, but insist she only showers on Sundays? Today must be Sunday, because it's time for her shower! Does she keep trying to leave the house at odd hours to go to school? It's summer vacation, she doesnt have school today! This approach is usually more effective than arguing when it comes to getting the person to cooperate with essential hygiene care or redirecting inappropriate behaviors.