"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?"
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.