"quieres comer" forms a phrase where both verbs are linked together, as this is the case we conjugate the first verb and the second verb remains unchanged, it is similiar to english where we would say "He could go", "could go" forms a phase so we do not conjugate the second verb i.e. we dont say "he could goes"
It depends on the situation again. “Do you want more bread? (or something else) Or shall I get you dessert?”
“Do you want to eat more bread?” (or do something else with it) Or shall I save it for later? (I am not offering you more bread to take home to your dog.)
In Spanish they can also say “¿Quieres más pan?” which would actually be “Do you want more bread?”
The situations in which an English speaker would use "Do you want to eat more bread?" are few and far between. Learning languages is not about punching in exact word for word translations, it should be about learning how to COMMUNICATE in another language. In the overwhelming majority of situations "Do you want more bread?" or "Would you like more bread?" would be used over "Do you want to eat more bread?" which sounds stilted and unnatural in English. Duo should accept as correct the translation that would be the most natural and widely understood instead of just literal translations.
"Do you want to eat more bread?" sounds ridiculous for me if it’s not only a sarcastic question suggesting that the other person like a child makes something wrong with the bread e.g. creates bread balls. IMHO it should be "Do you want more bread?” In a situation "save it for later" I’d use "Do you need more bread?"
No, "need" and "want" are different and both exist in Spanish also, just as adding "to eat" exists in English as well as "comer" in Spanish. What you personally would do not withstanding, it still exists and I do use it even if you don't. The more natural sounding version is also used in Spanish. "Do you want more bread?" = "¿Quieres más pan?"
"Comes" is the conjugated form of eat used for "tú" informal singular you.
"¿Comes más pan?" would be "Do you eat more bread?" or "Are you eating more bread?
This sentence has another verb "Do you want to eat more bread?" Notice that we are using the infinitive form "to eat" after the conjugated form "want". So it is "¿Quieres comer más pan?"
"comer" = "to eat"
This is not poor English. It is more specific. I could also ask before I go to the store "Do you want more bread?" meaning "Do you want me to get more bread?" or since I am going to the store to buy food it can mean "Do you want me to buy more bread?"
So it is not a tautology as it can mean something else. Context is everything.
Yes if you put the accents on tú and más, and “¿Quieres tú comer más pan?” would also be correct, but Spanish usually drops the subject pronoun so you would be emphasizing “you” as opposed to someone else. “Tu” without the accent actually means “your”, so without the accents, no, it would not be correct.
If you were translating from English to Spanish, then it should also be accepted as correct. Often though, I might say this to a family member and so I would use quieres.