Question #1: Do angels sometimes inhabit a body for the purposes of being seen or are they seen as silhouettes?Angels do not inhabit bodies because, by their nature, they are not made for that sort of thing. Angels are a type of spirit that does not reside in a body. I'm not even sure that one could even if he wanted to. So, if an angel wants to be seen by man, he simply takes on the image of a man. He appears like a man, even though he is not.
Q #2: In which case, what would be the differences in the functions of angels and saints?Well, their functions are the same in the sense that they both live in perpetual worship of the Lord. Their functions are different in that angels have the distinct mission of "messenger." The word angel comes from the Greek word angelos which means "messenger." So, angels often have the task of delivering messages from God to His people. Human spirits were not made for this purpose.
What makes the spirits of the saints unique is that they were made to be the animating principle of a human body. In other words, they were made to give life to a human body, and to be the force of the human person that thinks and wills. Thinking things and willing things, these are actions of the spirit carried out in a human body. Angels, of course, were not made for this purpose.
Q #3: What are the differences in their miracles?There is no difference. Also, note that no angel or saint causes a miracle to occur by his own power. Angels and saints aren't gods. Instead, whenever He wills it, God gives them a special grace or power that allows them to perform the miracle.
Q #4: Angel Gabriel, for example, provided a message. Could he have taken a human body or was he seen in silhhouette?I'm not sure what you mean by "silhhouette" but no, he didn't take on a human body.
Q #5: What is the difference between an angel spirit and a saint spirit?An angel spirit is was made to be pure spirit, a complete and formless being. A saint spirit is formless too, but it was made to be with a body, to give that body life, to be its animating principle. It is in a sense incomplete until it is reunited with its body again.
Q #6: Is it correct that to be a saint; one has to know their calling, create 3 miracles, and be canonized?To be a saint, you just have to live a life of grace. You have to be holy. That's why Paul calls some of the members of his churches "saints" even though they haven't died yet.
Now, to be officially declared a saint by the Church, yes, three miracles have to be attributed to your intercession after you die. This is because, if a spirit responds to the prayers of people on earth, then this means he is in heaven, and if he is in heaven, then that means he was a saint in this life. Of course, when deciding on whether or not to canonize a person, the Church also investigates the life of the person to see if it was marked by holiness. Also, when the Church canonizes a person, that doesn't mean that she just made that person a saint. The person was already a saint, it's just that, in canonizing them, the Church confirms that fact and says that it is ok to pray to and to venerate the person in question.
Q # 7: What does one have to do be an angel, in spirit?I don't understand this question.
Q #8: How are angels chosen and/or created?The Lord created the angels out of nothing, like He created all things. Every one of the angels that exist in heaven were created all at once, the day the Lord created the heavens and the earth.
Q #9: Are angels spirits that were not ever born as people?That is correct.
Q #10: So, when saintly people die and become saints, are their spirits angelic?No, because the spirits of human beings are different from angelic spirits. They are a different type of spirit.
Q #11: So, would angels and saints have similar spirits in heaven? If so, what are those similarities and differences?Well, they are similar, but they aren't exactly the same. See my answers to Q #2 and #5.