Welcome to the forum. Here are a few initial comments:
Q1 - Considering the materialist and empiricist characteristic of Epicureanism, how did Epicurus manage to arrive at the certainty that "nothing can be created from nothing". Indeed, his argument makes sense, but has this been empirically observed to be raised as a universal and indubitable certainty?
A1 - The short answer would be combine both observation and logic. First he trusted his observations that nothing that he experienced, or that reliably reported to him, established that anything ever comes from totally nothing. He also added a series of deductive questions and answers about how things would be different if things DID come from nothing, and likewise observe that those things do not happen. The longest list of these is in Book 1 of Lucretius, with some being included in the Letter to Herodotus. I understand that you are using a translator so "universal and indubitable certainty" may be overly broad, but it's also true that Epicurus used a 'reasonable' standard of proof and did not accept logical absolutes as required for holding something confidently to be true.
Q2 - Considering the argument that all things need a cause in order to exist, this opens up an opportunity for the existence of a creator, or creators. How did Epicurus, empirically or rationally, come to the conclusion that their interference in our lives is unlikely? How did he come to the conclusion that the gods he mentions in the letter to Menoeceus are blessed and only relate to their fellow men?
Your questions of course touch on why Epicurus was very different from the Stoics, and why the Stoics denounced Epicurus, because Epicurus rejected all contents that there is anything above or outside nature, or that nature was created by supernatural gods, and at root Stoicism is a "supernatural-based" outlook on nature and how to live.
A2 I don't think that Epicurus accepted that "all things need a cause in order to exist." There is no reason to accept the possibility that the particles have not existed eternally, and there are sound logical reasons for believing the opposite, again as listed in Book 1 of Lucretius and the Letter to Herodotus.
As to the interference of the gods, Epicurus held that any gods that exist would logically be completely happy and deathless in themselves, and they would accordingly have no reason to be interested in making trouble for enemies or rewarding friends, because making trouble or rewarding friends is a characteristic of beings who would not be complete within themselves.