You havnt failed, you will do your best for your mum but you cannot just put everything you feel aside to be strong for her, you need to be strong yourself ,which means looking after yourself as well.
If you feel you can talk to your dd in his room then changing the bedding doesnt take him out of the room or wash him away - but on a practical level did he have a favourite aftershave or soap that could bring back that sense of him being there - are his clothes still there - i found wearing a big cuddly fleece of my husbands comforting it had been washed but it didnt matter it still felt like his.
I understand why you feel annoyed with your dad but you know that he was trying to protect you in what he did - all any of us can ever do is what seems right at the time but we make those decisions only with love and thats really what matters, more then whether we agree they were the right ones
Grief is a long journey, as is caring for some-one with dementia and you are just starting out -you will only ever do the best you can for your mum on that journey.