Is it true a guardian angel is assigned to every person? If this is so, do our prayers go right to God, or through our guardian angel?
It is taught by the Church that each believer has a guardian angel. The Catechism says "Beside each believer stands an angel as protector and shepherd leading him to life." (# 336). And this fact also flows from what Jesus says. "See to it that you do not despise one of these little ones, because I tell you, their angels always behold the face of my Father in heaven.” (Matthew 18:10)
Regarding the second part of your question, it can reasonably and rightly be argued that angels do serve as intermediaries in our communication with God. The very word “angel” means “messenger,” and it is clear that God often mediated his message to us through angels.
Regarding our prayers going to God, it is not unreasonable to presume that angels serve in some way to mediate these messages. However, it does not follow that God does not know or hear us if we don't tell our guardian angel something, or that the only way a message can reach God is through his angels. God is omniscient, knowing all things in himself.
Further, while Jesus does not forbid us to pray to our angels, when he teaches us to pray, he tells us to pray to our Father who is in heaven. Hence, though angels may help to serve as intermediaries for these prayers, we ought to have our attention on God. Consider, for example that if we spoke to someone through a translator, we would not tell the translator to say something to the other person, we speak directly to the other person and simply allow the translator to do his or her work.
Hence, exactly how the angels serve as intermediaries in our prayers to God is somewhat speculative, but the point is to focus on God and pray to him in a natural way. To whatever manner and degree God has our angels serve as intermediaries is really not important for us to know. What is important is that God hears us, that he knows our needs, and what we say, and that he loves us.
I read in a certain spiritual work that at the Last Supper the Holy Eucharist returned to being bread only when received by the traitor Judas Iscariot. Is this true?
No, when the bread and wine are consecrated and become the Body, Blood, Soul and Divinity of Jesus Christ, this effects a permanent and substantial change, such that the Body and Blood of Christ will not go back to being something else. Further, there's no reason, biblically or theologically, to hold that the case of Judas would be any different.
"Building our Catholic faith one question at a time."