My mom went in2 cardiac arrest & brought back but on life support -before her brain died completely could she hear me telling her I love her please?

Yes. It is likely that she heard you. I guess it was her time. I tell the loved ones and staff that are within earshot of my patients who are in a coma, to assume that everything they say can be heard and understood. If they cannot hear then nothing is lost through such acts of kindness.
Completely . unscientifically, I think as long as your Mom had any brain function, she sensed your presence & your love on some level. If it makes you feel better, watch how often the " Long Island Medium" tells people exactly what they did at their loved ones' last moments, allegedly having been informed by the deceased. Unfortunately, I don't know how one would confirm or refute that, but it's comforting.