As you can probably tell by now, the way to determine if the "prophecy" stated by the missionary is what it purports to be one merely needs to go back to the original source and compare it with the "matching" passage in the Christian bible. What does B'reshit / Genesis 26:2-5 really say -- and then what does the Christian bible use it to say?
"G-d appeared to [Isaac] and said, 'Do not go down to Egypt. Remain undisturbed in the land that I shall designate to you. Remain an immigrant in this land. I will be with you and bless you, since it will be to you and your offspring that I will give all these lands. I will thus keep the oath that I made to your father Abraham. I will make your descendants as numerous as the stars of the sky, and grant them all these lands. All the nations on earth shall be blessed through your descendants." Br'eshit / Genesis 26:2 - 5.
Genesis (B'reshit) 26 is about Isaac and the Philistines. The first sentence (which the missionaries conveniently ignore) says "There was a famine in the land, aside from the first famine in the time of Abraham. Isaac went to Abimelekh king of the Philistines in Gerar."
So this is about a familne during the time of Abraham -- it is not a messianic prophecy. Isaac goes down to Egypt. G-d tells Isaac to go to Egypt and says to him "I will be with you and bless you, since it will be to you and your offspring that I will give all these lands."
As in the last one we have offspring (plural) -- not a prophecy about one person (Jesus).
Hebrews 11 is not purporting to be a prophetic fulfillment -- so this particular "prophecy" is a defnite reach by the list maker. Hebrews 11 speaks about the faith of various persons in the T'nach (Jewish bible). Hebrews 11:17 - 19 discusses the faith of Abraham as part of an overall discussion of faith by Abel, Enoch, Noah, etc. The passage about Abraham discusses the Akeida (the binding and near sacrifice of Abraham's son, Isaac). Neither B'reshit / Genesis 26:2 - 5 or Hebrews 11:17 - 19 have anything to do with the messiah (redeemer).
B'reshit / Genesis 26:2 - 5 is simply saying that through Abraham and his son, Isaac, the nations will be blessed. It has nothing to do with a single Jew (the messiah), but rather to Isaac's descendants (plural). וּֽלְזַרְעֲךָ֗.
How does the list maker (claiming this is a messianic prophecy about Jesus) get a messianic prophecy out of the line "Remain an immigrant in this land. I will be with you and bless you, since it will be to you and your offspring that I will give all these lands. I will thus keep the oath that I made to your father Abraham."?
Notice that part of this line is that G-d will give Israel (the land) to the Jews -- and yet a mere 40 years after Jesus supposed death the Jews were thrown OUT of Israel -- for a mere 2000 years!
In other words, Jesus FAILED to do this (if this was supposedly a prophecy about him).
But back to זַרְעֲ - (zera) aka offspring, descendants, seed. . . זרע [zera] is a collective noun, which means that it has a singular form, but it includes all of the people who come from a parent. It is similar to משפחה [mishpachah - family]. Although, family is a singular noun, it refers to more than one person. Jesus was supposedly just one person (at least in THAT physical form) -- but זרע is not used to speak of singular people unless that person is identified by name -- and the name "Jesus" is not used in Genesis 26!
Only by relying on the ignorance of his followers does the author of Hebrews dupe his readers. This has nothing to do with Jesus and it is not a messianic prophecy. This passage does not point to the messiah, and it most certainly does not apply to Jesus -- even when contrasted with Hebrews 11:17 - 19.