I see you know French; my French is a bit rusty, but I think it might help:
There are some nouns in English--especially foods, drinks, and abstract concepts--that can be both countable (which will take "a [singular noun]" or "the [plural noun]," and be pluralized) or uncountable (which can only take "the [singular noun]" or just the singular noun with no article at all; it cannot be pluralized), and take on different meanings depending on which form you use.
Commonly, the countable form suggests different varieties, while the uncountable form suggests a individual item which will be divided upon serving. So, "the bread" or "bread" just means all bread as a whole, or some unspecified amount of bread. "A bread" or "the breads" suggests that we are talking about varieties of breads.
I think, in French, you often use de la/du for our uncountable form. You cannot drink une eau, but you can drink de l'eau. Or how je mange un poulet suggests I eat an entire chicken, while je mange du poulet means I eat an unspecified portion of chicken, but not one whole chicken.
Since "bread" doesn't have "a" and isn't pluralized in this case, it's the uncountable form. This means someone is using the largest portion of some amount of bread. Imagine there's three people in a house; one of them eats a single slice of bread, while the other two makes twenty sandwiches. They are using most of the bread.
If you said "they are using most of the breads," it suggests that they have a bunch of different varieties of bread, and they are using most of the varieties.
Yes. It's completely normal and standard. Maybe because bread is (still) mostly bought in loaves of many pre-sliced slices, not in a form that someone would commonly use more than one loaf, "bread" is usually an uncountable. It would be strange and pedantic to specify that they use the most slices of bread, or the greatest quantity of bread, rather than the "most bread." If you had a limited quantity of bread, you might specify "most of the bread," to emphasize, "most of the bread that we currently have." But again, if you buy some supermarket loaves (which last for a couple of weeks!) you may not commonly run out of bread. So, basically, there are a lot of practical cultural realities about "how bread works" that probably have shaped the language.
I notice it because in our house, we (often, or at least, until it runs out) have "the good bread" (from the excellent bakery a block away) in the breadbox...and we also always have a loaf or more of "bread" in the freezer, for toasting.
That would refer to the frequency, not to the amount. Someone who makes 200 sandwiches on just one day is using more bread than someone who makes a sandwich three times a day for a whole month. 200 is more than 90. However, the person who's making sandwiches three times a day is using bread "the most."
I think all comparatives follow the same rules of standard adjectives, so -e is used in declined forms.
Hij is het sterkst : He is the strongest. Since it's not directly modifying a noun, it doesn't get the -e.
Hij is de sterkste beer dat ik ken: He is the strongest bear that I know. Since strongest directly modifies bear, it gets the e.