Your child live in peace in oblivious bliss, even if mostly alone.
OR
Get involved with the 7 elemental dragons that ruled over the spirits and all the countries that will try to manipulate her for their own benefits once they realized she can do that?
The 'anti-beast' barrier is an actual barrier which is meant to keep Sena from being detected, since as soon as she left that area there's this chapter "I'VE FOUND THE PERSON" immediately.
The mom didn't have long to live, and probably didn't want to force her daughter to face the heavy destiny as a 10-14 years old, that's why she prepared the letter but also let Sena lived as a shut-in, so that if Sena choose to remain hidden, she can, and if she choose to come out, there's also a plan for that.
Also, don't forget that she was a 10 years old when the incident happened.
If a child that young run into the person that left them scared from the angry shouting earlier, there's a non-0 chance of her going into panic and use her magic and unintentionally murder the people trying to come and apologize. She'll DEFINITELY permanently become shut-in if she ever accidentally killed someone like that.
As for telling her the townspeople want to apologize...well, not ALL of the townspeople are like that (see back when the old man was going to push himself to deliver her stuffs) so unless she is willing to face some criticism when she finally come out, it's better not to reopen the old wound. Not to mention when the child is in self-loathing mode like that, trying to convince them would actually have opposite effect.
And remember, Sena went to that bar without knowing the townsfolk were going to apologize, she made up her determination to face the criticism before going there and learning the truth.
Yeah, compare to a normal mom with a child who doesn't have world-changing destiny, she'd be a bad mom, but when considering that information, it's hard to say she did the 'wrong' thing by trying to give her daughter a peaceful life given that the mom likely suffered a lot before settling down, seeing as there's no dad mentioned and the knowledge mom has shown in hindsight.