11) What happens to a person who passes away who is Shomer Habrit versus someone who is not? Does the person who is not Shomer Habrit have more suffering in the grave? Does he spend a lot of time in Gehinnom? What is Gehinnom like anyway? What information do you have about the afterlife?
Nobody knows what the after life is really like. The Torah hid this from us so that our divine service should be more for the sake of heaven and less from the fear of punishment or for the sake of the great reward that we will receive in the next world.
However, the Gemara and the holy Kabbalistic books drop hints about the punishments and rewards that await the evil and the righteous in the next world. It is written that those who have guarded the bris will be protected from Gehinom by Avraham Avinu himself, who was the first to fulfill the Mitzva of circumcision. In the Zohar it is brought down many times how the angles of vengeance, and the arch-angel Dumah, who is appointed over Gehinom, have no power over one who has guarded his bris in this world. On the other hand, one who did not do Teshuvah in this world, will be chased by his sins in the next world. The Talmud brings Pesukim that imply that every sin we do will be tied to our souls and torment us in the next world if we don't do Teshuvah on them. The sins themselves will testify against us and cause us great shame in the world of truth. Also, if one did not work on himself in this world and he is steeped in physical desires, his soul will not be able to break free of the body after he dies. He will be tied down to his body until it has totally disintegrated into dust, before he will even be let into Gehinom. He will also feel great anguish that he can no longer indulge in the physical pleasures that had been so much a part of him. However, the soul of one who has elevated his physical desires to the service of G-d, will break free of the body right away after death, and rise straight to its honorable place in Gan Eden.
12) What do you mean that he will be tied down to his body for a long time? Does the dead body feel all the animals that are eating away at it after death? What exactly is happening when we refer to the suffering of the grave?
The Talmud writes that the worms eating the flesh after death are as painful to the soul as needles in the skin (of a live person). However, I believe that this applies only to those who were tied to their body's desires in this world. One who has done Teshuvah and learned to use the desires of this world only for divine service will not feel this pain, which is known as "Chibut Hakever". There is other suffering that happens to sinners in the grave, such as "Kaf Hakela", where the soul is chased from one end of the world to the other by tormenting angels created by his sins, but since I am not a Kabbalist, I can't elaborate on that which I don't know too much about :-)
Let's just hope we don't need it.
13) What does it mean when it is said that a person has two names, one given to him by the Sitra Achra? Does a person have to find out what this name is?
The Holy Ohr Hachaim Hakadosh on Parshas Korach explains that every person's name holds a double meaning, as well as a double potential. The name a person was given at the time of his bris ("Ploni Ben Ploni") has a holy meaning hidden within it that symbolizes that particular person's spiritual strengths. At the same time, if a person does not realize his potential and follows his evil inclination G-d forbid, then that very same name--including the names of his father and grandfather--hold a hidden meaning that symbolize this particular person's spiritual downfall and suffering. That is why Chaza"l explain each of name "Korach Ben Yitzhar Ben Kehas Ben Levi" in a bad light, even though his fore-bearers were greatTzadikim. Because, once Korach had misused and destroyed his lofty spiritual potential, his name, along with the names of his entire lineage, became symbols of his particular spiritual downfall.
14) How can a person find out what their particular Tikkun is in this world?
I have heard it said in the name of Tzaddikim, that one can know what his particular Tikkun is in the world by seeing in which areas he struggles the most. It is in those areas in particular that his soul came down to this world to fix.
15) What is the best way to destroy the damaging angels and demons if we have done something wrong to bring them into being?
There are many types of Tikkunim brought down in the holy books, ranging from all types of prayers, like the Tikkun haklali of R' Nachman (Tip #9), giving a lot of charity, immersing in the Mikva (Tip #10), and crying with real tears. But the biggest and most successful tikkun of all, especially for our weak generation today, is simply doing a real Teshuvah and not going back to one's old ways. Learning to guard the eyes and giving up the lust in one's heart, these things are the biggest tikkun for all the bad angels that one has created with past misdeeds, and indeed, through a true Teshuvah, these angels become transformed into holy angels.