You address several rules. If there is in the set of foods one food from the 5 species of cereals, he is the Ikar, but there is exceptions, if this food function to sweat the dammaging effect of one other food, or is only to help against suffocation. The rule of 5 species of cereals is a part of the svara of Ykar and tafel, essentiality and contingency.
- Mishna brachot (in gemara 44a) taught the principle of ykar and tafel. (By the bye,from this principle is derivated rules of rov (majority).)
זה הכלל כל שהוא עיקר ועמו טפלה מברך על העיקר ופוטר את הטפלה
Concept of lelafet et hapat (the condiments of bread) (in gemara 41b) says that foods that are eaten with a piece of bread are tafel the bread.
אמר רב פפא הלכתא דברים הבאים מחמת הסעודה בתוך הסעודה אין טעונים ברכה לא לפניהם ולא לאחריהם
Gemara regarding the rule of 5 species of cereals (36b), they adress a mixture or a combination of essential food and an accompanying food. The ykar of the mixture is not necessarily the component that is in greater amoun., if one the components is a 5 cereals specie, it's the ykar.
דרב ושמואל דאמרי תרוייהו כל שיש בו מחמשת המינין מברכין עליו בורא מיני מזונות
Apparently it's an exception to the second rule, but in reality a case that shows the deep sense of the second rule. (from the upcited mishna 44a). When the food eaten with a piece of bread is what you want to eat, but you can't eat it alone because it's too strong, e. g. too salted. In this case, the bread comes as an attenuator and is tafel.
הביאו לפניו מליח תחלה ופת עמו מברך על המליח ופוטר את הפת שהפת טפלה לו
Tosfot says that this case comes to teach that bread is not always the essential food even if you eat the second food with the bread (as a sandwich for instance)
ואף על גב דאשמעינן דפת פוטר את הפרפרת איצטריך לאשמעינן
דפעמים דפת טפל
- The concept of chashuv is generally used when two foods with the same beracha, to decide on what to bless.
So we explained some principles regarding ykar and tafel, you addressed in the text of the question.
- But your case address the din kdima, precedence, of brachot. A case in which you need anyway to make 2 Berachot, on what to begin. Gemara 41a take an example of bracha Haadama and bracha Haets. There is an opinion in Gemara that such a case is in machloket between Rabbi Yehuda and rabanan in mishna (the Mishna is in 40b on Gemara). Rabbi Yehuda says to begin by min Shiva (the 7 species quoted in Tora in the shevach of erets Israel), and rabanan say that to begin by the favorite. In halacha OC 211.1 there are several opinions and a lot of cases are discussed.
For the kdima, e.g. between shehakol and mezonot, the bracha of mezonot is more specific, and bread is from the 7 species, so, it's before shehakol. Ha motsi is before all. See the whole siman 211 in SA OC
All this if you want to eat the both at the same time. But if there is an order in the art of eating, this order is prominent.
It's normally to eat the sandwich before.
- Regarding the after bracha, if there is not a case of ykar and tafel, to globalize in one after bracha, the question is if there is a notion of seuda, if there is a kviut seuda, the after bracha is birkat hamazon. If not, there is two after bracha anyway.