We don't talk about Bruno: Delores actually told Mirabel vital information, but Mirabel ignored it

The key here is addressed in the picture - "Do you understand?"

Mirabel didn't ignore it, she indeed did not understand.

She didn't remember Bruno, she was indeed being given misinformation (especially by Camilo), and had a hard time assimilating what Dolores was truthfully telling her.

Everyone assumed that Bruno 'had left them'.

Interestingly 'he left us' is a common synonym for: 'he died', especially with a Christian context. It's often used in eulogies or funeral sermons.

The whole behaviour of the family is common when a family member committed suicide. The anger and refusal to talk about it and putting so much emphasis on the insistence that the person didn't care for the family or just left them behind is very typical for that assumption.

It's also corroborated by the way Julieta talks about her brother.

So listening to Dolores doesn't help Mirabel. She thoroughly assumes that Bruno is gone, maybe even dead. Since Dolores doesn't directly say: hey, he's in the walls, I can hear him everyday, he never left, the information she gives her during the song is not revealing at all, it's just more confusing.

Well, Dolores does give her a much more straight hint right at the beginning, when Mirabel asks her if anybody worries about the miracle she says: just you, and the rats in the walls.

Mirabel then saw Antonio next, while he spoke to the animals. So she could have asked Antonio to speak to the rats and ask them what they know about the Miracle, since he had already proven that you can talk to the animals and they can give information.

Later again she gets the information that Bruno kept rats, and she sees rats again in Bruno's room, so it's beyond me why she never asked Antonio to speak to the rats and ask them what they knew.

I personally think that was the bigger oversight.

Her name is spelled Dolores

Dolores was interesting and I would have wanted to learn more about her personality, but it’s possible that whatever personality that was made her untrustworthy.

There isn’t really anything mirabel could have done with that information. It hardly made any sense. If someone in your family disappeared would you ever guess that they are in the walls? She probably thought Dolores was crazy or having flashbacks, that’s what I would have thought. Also Mirabel didn’t need to know why the community turned on him. she just wanted to know how accurate his visions were because well ya know she just saw a about herself that didn’t seem good.

Dolores could literally hear people getting lost in the forest from what I understand. Her gift was such a ridiculous hearing ability that she could eavesdrop on the village/town when Casita was on the edge of it (I think) so Bruno could've totally been in the middle of the forest or even in a cave in the mountains.