This is a great series of questions!
Such a low mass black hole (BH) could have originated from a few possibilities: 1) a result of stellar evolution (the resulting black hole mass depends fundamentally on the initial mass and metallicity of the stellar progenitor, among other things); 2) a star collapsed into a neutron star which can accrete matter from its surroundings until its mass overcomes the neutron degeneracy pressure and forms a BH; 3) a result of a binary neutron star merger.
- Is it theoretically possible for a small black hole like the unicorn to revolve around a giant star like UY Suit?
Yes. In the three cases above, the newly formed low-mass BH could be captured gravitationally by a nearby star, for example if it forms in a dense stellar cluster.
As a different example, consider the life of a stellar binary, a pair of stars that are gravitationally bound to each other, in which they evolve in a kind of kindred evolution where the development of one can affect the development of the other. Generally, one star will be initially more massive than the other, and at the end of their stellar lifetimes they collapse into compact objects - let's assume their initial masses are high enough and metallicity is low enough that a BH forms when each collapse. So, generally, the initially more massive star will collapse into a BH first, leaving the possibility that the binary is composed of a BH and a star. In this system, the bodies are mutually orbiting each other, but from the star's frame of reference, the BH is orbiting it. You can extend this thought experiment by adding more companion bodies, but it that case the bodies would orbit about the common barycenter.
Observationally, an example is a high-mass X-ray binary system, where a compact object, such as a BH, accretes matter from a stellar companion, and the BH can be thought of as orbiting the star. This system is such an example
2- Will such a star be stable?
3- Will the blackhole take mass from UY Scuti and form an accretion disk?
4- What will happen if both of them collides?
Continuing with our example of the stellar binary: the answer to your question essentially depends on whether the stellar companion of the low mass BH will fill its Roche lobe, which depends on various things. Basically, the Roche potential defines gravitational potential of a classical binary system, and the Roche lobe is the region around a star in that binary system within which a particle is gravitationally bound to that star. So, as a star ages it generally expands, and by the time it evolves into a late-stage gas giant, its radius is huge. To first approximation, the radius of the star's Roche lobe is proportional to the binary separation (and it depends on lots of other things, like the orbit's eccentricity, the star's rotation rate, etc...). Thus, if the separation is small enough, then the companion star can fill its Roche lobe, which causes matter to pass through the first Lagrange point to the BH - this is known as "mass transfer" in binary evolution theory, and is uncertain and complicated, but that's the basics of it. The mass transfer can be stable, dynamically stable, or dynamically unstable (also known as common envelope evolution because the donor star's envelope engulfs the entire binary). Generally, an accretion disk may form in this process.
If the mass transfer is dynamically unstable (meaning the BH's accretion rate cannot keep up with the mass transfer rate from the star), then common envelope evolution may cause the binary to merge as the viscous friction of the binary's motion through the common envelope expels the envelope by shrinking the binary separation. Common envelope evolution is observationally unavoidable, but the physics of it is still uncertain. If such a merger/collision occurred, the BH would likely tidally erupt the star (though this is also complicated!).
