Your pup is adorable. Not sure if the double at 12 weeks would work since Bichon are a little larger dogs and may follow a different growth pattern. Sounds like you are off to a good start with training. Potty training takes time. I started right away with outdoor training, which I think is easier, although come winter I'm going to be wishing she was pee pad trained
It will take some time to get them used to brushing. Lexi HATED it and I thought I would never see the day she would let me brush her, but the day did come.... months later! For the bad tangles I just cut them out. It might look funny in some spots, but it will grow back and I never have any trouble with tangle anymore. Good luck!