"I eat lunch at the cafeteria."
This is part of Keigo, that is, polite speech. お is pre-pended to some nouns as a show of respect to your listeners. https://www.wasabi-jpn.com/japanese-lessons/the-honorific-form-the-humble-form-and-the-polite-form/
It's an honorific prefix for politeness. I believe お is used for words of Japanese origin, while ご is for words of Chinese (more accurately, Sino-Japanese) origin.
No, に is usually only used to mark locatiin of action for certain movement verbs like 'to go' 'to come' 'to return to' etc. You COULD use に but it sounds weird to a native and more implies 'I ate in, or somewhere in the area of the chow hall' i mean, if you're unsure of where you ate...?
に is more of a stative particle, it states where you go to, where you are, where you are from, where something is, etc.
で is somewhat more active in the sense that you are stating where you are doing such action which in this sentence it refers to the verb 食べます.
In a later lesson I've seen it described as being kind of like "I use the cafeteria to eat lunch"
Not necessarily. De is also a place indicator, and it works in this kind of sentence.
De is a marker that indicates where something happens. In this case he eats lunch at the cafeteria. However Ni is used to indicate where something is
Are both レストランでひるごはんを食べます and ひるごはんをレストランで食べます acceptable? Or is there a correct sentence order?
This works, from my understanding. The particles do the work that word order does in English.
を usually marks an action I believe, and restaurant does not fit to that particle. I might be mistaken here as my only experience is 6 months study in Japan so it would be great if someone could confirm this.
I understand this sentence as "I eat lunch via the cafeteria". This helped me understand which particle I needed to use.
I hear no difference in pronunciation between 'を' and 'お'. Is that normal?
I wrote "おひるごはんをしょくどうで食べます". Duolingo still marked it but I just wanted to know if this is actually acceptable.