I have answered all of these questions elsewhere in similar posts, but I will do so once again:
The UC sold by BALG, has a mediator spirit bound to it, that helps to make evocation smoother, and the summoned spirit more agreeable. His name is Malzaz Hyroth, and he is respected by all spirits. However, you still have to be able to bring the spirit down into physical manifestation yourself. It doesn’t do the work for you.
It’s actually referred to as an astral evocation because you are not bringing the spirit directly down to the physical realm, but viewing it on the astral through your mirror. The book Summoning Spirits by the author Konstantinos has a good explanation of astral evocation with a mirror.
It depends on the system you are following. In traditional Solomon magick, the mirror is placed inside the triangle, but in most other systems it is placed on your altar, along with your other tools.
Again, it depends on the system you are working in.
EA uses a circle and a triangle in his system outlined in his books Evoking Eternity, and Works of Darkness. He has now evolved in his practice to the point that he doesn’t need one, and he prefers to let the spirit manifest wherever it wants to because he’s powerful enough to control it if it gets out of hand.
The prime use of the triangle is simply as a place for the spirit to manifest. In Solomon magick, names of power are written on the triangle to bind and constrain the spirit, but those are not necessary. I’ve always thought of the triangle as putting out a chair for a guest. It is simply the place set aside for the spirit.
You can use the UC alone if you prefer. I do. Just be prepared for the spirit to manifest wherever it chooses, like behind you, or beside you, or even continually move around, never standing still to talk.