I'm so sorry your mom is going through this awful disease.

I've never heard of a tongue flap before, so I can't comment on that. I have had a glossectomy and had a flap taken from my wrist. I do have a speech impediment but I have had more of my tongue removed than your mom is going to have. Learning to speak again can be challenging. The flap is sewn into the mouth and doesn't move. There is no sensation from it either. It can also take a while to get the strength back in the arm after surgery.

Not having a flap will cause issues for eating and drinking. The tongue goes down deeper than you realise. Once that has been removed then the reconstruction is there to fill the void to prevent food, water, medication etc for falling into the gap. If it's left there it can cause issues such as infections.