Yes, it is exhausting and scary and is constantly pulling you in different directions. Getting all the legal documentation out of the way was a wise move on your part
That will make your life easier in the long run. You can't assume that just because you were married for so long that all will be ok... the paperwork you have proves it is ok
Yes, your daughter does need you. It is hard for us to grasp what is happening, and it has to be even more confusing for a 15 year old. My best advice to you is to make sure you carve out time for your daughter. Talk to her. See how she is feeling. What are her fears. If necessary find a trusted friend, professional, or other adult that can mentor her. Give her a soft place to fall when she is overwhelmed. But most of all be there for her. Don't let all this take away from what can be between you and your daughter. And I will say again, you have to take care of yourself as well!!
It is typical for someone with dementia to want to be close to the person that they recognize. A large percentage of his world is a blur of confusion but obviously you are his stability. As annoying and exhausting as it can be, there may be a time that he will not be mentally of physically able to hang right behind you. You will learn patience
he is not trying to be annoying. He is just trying to find something that has meaning and familiarity. On those days I got so very frustrated with Mom or Dad... I would try to put myself in their head. That would ground me and give me the patience I needed.
Prioritize what you need to do. Make a list if necessary. Check off what is done so you will know you are making progress. If there are things that can slide... let them! In the end nobody wishes they had worked harder or had more... they just look at the relationships and love in their lives