1.Actually how deaf is Shouko ?
Actually, it's already been stated in this question. Well, just some part of it that answered your question. But from your state on the question above comes out a new topic to discuss. But there's another topic to discuss deriving from your question.
If it wasn't the case when she was still a child, has her sense of
hearing been improved until now to the point in which Shouko's able to
understand people clearly when they talk slow enough? If I'm not
mistaken, depending on the type of deafness, it is possible to
stimulate the regeneration of the hearing ability through medicaments,
clinical interventions or sometimes even without any interventions.
Well, this is kind of true, but in chapter 32.5 there was a scene in which Shouko practiced a lot to improve her sign-language with her grandmother. She is doing this because of Yuzuru. Here are the pages in which Yuzuru confronts Shouko with her voice. (Read from right to left)
Shouko : Uyuu!(Yuzuru!)... Ah oohh(???)
Yuzuru : I don't think you should speak in front of other people.
What Yuzuru told her has (somehow) motivated Shouko to improve her pronunciation and sign language so it will make her sound not so bad for other people. You can see her practicing a lot with her grandmother. While Shouko was learning the sign language with her grandmother, we can see that they faced each other. I think the reason behind it is that Shouko wants to learn to understand people talking normally and how they pronounce certain words by reading their lips. The result of those practices can be seen on page 3.
Here, i.e. after the time skip, she's almost able to say "Yuzuru". Her pronunciation has improved a lot. You can mostly understand what she's saying, but it's neither grammatically correct nor phonetically correct.
And on page 6 you can see her practicing JSL with Yuzuru just like her grandmother did.
To answer your question
So, is it safe to assume that Shouko is and has always been able to
understand people when they're talking slow enough?
In the Ferris Wheel scene (chapter 27-28), Ueno said that she will speak slowly, AND she said that she will make sure that Shouko will understand what she's going to say.
The question is, did Shouko really understood what Ueno said?
In chapter 28 Shouko writes a letter for Ueno. This scene happened just after the Ferris Wheel scene. If Shouko didn't understand what Ueno said in the Ferris Wheel, she wouldn't have been working that hard to write the letter.
You can see the content of her letter on chapter 44, page 11-12. The content of that letter really fits with Shouko's conversation with Ueno back then, so it is likely her response to that conversation.
Based on this, it is correct to say that Shouko did understand what Ueno said in the Ferris Wheel.
2. Is it revealed why she is only wearing 1 hearing aid?
It's not revealed officially, yet. I don't know if it has been revealed, but a user said on the MAL forum which discusses a similar topic:
I'm surprised that no one knows the answer, considering how obvious it
felt. Near the middle of the movie, there was a scene with Shouko and
her grandmother talking with a doctor. After that scene, we see Shouko
crying with one hearing aid taken off. This basically means that
Shouko lost hearing capabilities completely in one ear. That's why she
doesn't use a hearing aid on that ear anymore.
I haven't watched the anime so I can't prove if this is true or not, yet in manga versions, they never mention anything concerning why Shouko only uses one hearing aid. And I think it's a bit different between the manga and the anime version since there was no scene in the manga in which Shouko and her grandmother were talking with a doctor.