1- Yes.

2- This is more subjective. These are ancient beings that should be approached with awareness of what they are capable of. They are beings of duality, but lean toward being intense to call upon. The inexperienced likely shouldn't be calling upon them, but it doesn't necessarily mean one would be harmed.
Many of them have the kind of power that you'd expect from immortals, as that's what they are. You should approach them with an awareness that anything you request of them becomes a contract. You're going to have to give them something in return for what they provide you, or to do things in a certain way that they wish.

Therefore, I don't think it would be appropriate to outright term them as dangerous beings, but I also don't think it appropriate to view them as generous benefactors to all who approach them. They are not to be trifled with and the outcome will depend on how they view you.
If you want to be respected by beings of this nature then I'd recommend doing as much personal work as possible with your own duality, getting accustomed to primordial energy, being open to having yourself challenged and wise to how exchanges with powerful beings truly work.
I wouldn't anticipate bonding with them; if they wish to have a closer connection with you it is typically because that would be of mutual benefit. Others may have a different perspective of these beings but this is the fairest to take, as you're optimally going in with established personal boundaries, an understanding of the nature of these forces and a willingness to embrace that nature.
Sure, the energy can have an unpleasant impact upon your life if you lack exposure/experience because it is a foreign presence entering into your familiar space.

3- I have not, and honestly wouldn't recommend going into a contract with these beings without already being capable of clearly discerning different energies or having reasonably functional psychic abilities such as clairvoyance. I'd work on that before approaching them. So if all of this was only because you want to petition one of them for that purpose, another option would be more suitable.
Theoretically yes, they can provide you with these abilities or help you to gain them, but if you want to go the route of striking a deal with an immortal for such a purpose I'd recommend one you have a more personal connection with.
However, it's important to state that there are no shortcuts to developing clairvoyance. Even if you were miraculously opened up to that ability tomorrow, you'd have to put in the work to maintain it.
You can tap into it without the help of outside forces; the key to that is self-awareness and that level of self-awareness can occur via a consistent meditation practice, for example.

Best of luck.

Thank you very much.

What I find strange is that there are many would-be magicians selling courses of goetia. There is a girl who says she talks to beings and she talks for a long time. I do not know if it is possible, nor do I know if she is in a position to know if she is really a daemon with whom she is talking.
There is even a magician saying that they are servers and they have to do what we want. and if you don’t do it, it’s because the ritual wasn’t perfect.
I myself have already bought a traditional goecia magic course. And I have also tried to do some ritual but it is not what I intend to be doing.
Here in my country now the goecia became very popular and each one created a way to practice.
blessings

1- can the demons of goetia be linked to an object?

Yes, they can be. Due to them being immortals, it's typically portals that help grant you quick access to them rather than a binding.

2- are beings or demons of goetia dangerous? I read many reports in my country and it has two strands, one that says that they can give terrible problems to those who invoke them. But there is another group that says that they invoke and talk to them in a good way ....

They can be very dangerous, some more so than others — namely for non-initiated Left Hand Pathers. But even for those who are initiated, they can still prove to be dangerous if one doesn't uphold respect towards them nor understand how that particular spirit functions. They can and will bring forth problematic outcomes as well, but depending on who you are to them it can be for personal growth of the magickian via hardships or it can be them attacking you in some sense. I feel that's an important thing to learn to distinguish, as if it's the former and you take it as an attack it can very easily become the latter. As Vipera said, any work you wish to accomplish with them will be taken as a contract. The Goetic spirits will ultimately being some form of transformation to the magickian. With my own experiences with them (namely as an initiated LHP magickian), there are several Goetic spirits who possess a calmer, loving aura to them while others have a much more intense, aggressive air about them. It really all depends on who you are trying to work with. Like all deities, they appreciate offerings. They also prefer it when one is clean and well-groomed in their appearance should you wish to invoke them. They see it as you taking your work with them seriously. It's important to be able to distinguish different energies as well — if they haven't any interest in you, they may not even show up. In which case, an imposter could easily show up in their place instead.

I would also like to point out Goetic demons are often very blunt and straight to the point. They will not sugarcoat the truth for you. That is not within their nature. So I wouldn't take the truths that they may tell you personally, or your ego may be the one responding to them instead. Learn to identity your ego, and work on un-learning Abrahamic subconscious imprints on what demons are and what they do. This will help you immensely, should you wish to work with them.

3- has anyone worked with a demon of goetia to obtain clairvoyance, apart from Astaroth?

I've worked with Goetic demons to help improve my psychic skills with various Goetic spirits. However, you should already have a decent grasp on said psychic skills if you plan on doing so. They're more likely to take you seriously then.

WRT your third question, I also feel I should mention they can teach you many abilities and skills you will likely have an incredibly difficult time learning about anywhere else. This is due to the fear brought forth from Abrahamic religions and beliefs on what they proclaim to be "forbidden knowledge," which in turn makes it hard to find reputable sources on them.