In June, I was diagnosed with Stage 3 cancer and had over 60% of my tongue removed. The amount they removed depended on the tumor....they kept taking little slices off until they knew they had removed all cancerous cells. It was rebuilt from muscle in my arm, but there are other areas on the body that can donate the muscle. My speech has only been slightly altered. I am clearly understood by everyone, and even got back to singing with my church choir. I didn't require any type of speech therapy at all.
Most survivors that I know can speak very clearly, and they had alot more tongue removed than what your mum is facing. I even know a man who had his entire tongue removed and rebuilt and he speaks very well.
The biggest problem I have deals with lack of saliva, but that was brought about by my radiation therapy. Your mum will be lucky to not have to go through that experience.
I want to warn you...the description of the surgery sounds much more graphic than what it looks like when it's all over. The scars I have are barely noticeable. People wouldn't know I had anything done if not for the gauze I have to wear due to a side effect that's still lingering on.
It sounds like everything is turning out great for your family. I will keep praying for you all so that it keeps going well.