When looking through the glasses of Darbyism, or otherwise called "dispensationalism", things become dim and uncomprehendable, although Romans 11 is clear, and give us the context of Gentiles. Gentiles are mentioned five times in Romans 11, quoted here for your ease of reference:
Romans 11 (ESV)
11 So I ask, did they (Judah among the Jews) stumble in order that they might fall? By no means! Rather through their trespass salvation has come to the Gentiles (and Ephraim among them), so as to make Israel jealous. 12 Now if their trespass means riches for the world, and if their failure means riches for the Gentiles, how much more will their full inclusion mean! 13 Now I am speaking to you Gentiles. Inasmuch then as I am an apostle to the Gentiles, I magnify my ministry 25 Lest you be wise in your own sight, I do not want you to be unaware of this mystery, brothers: a partial hardening has come upon Israel, until the fullness of the Gentiles has come in.
Genesis 48 (Young's Literal Translation)
17 And Joseph seeth that his father setteth his right hand on the head of Ephraim, and it is wrong in his eyes, and he supporteth the hand of his father to turn it aside from off the head of Ephraim to the head of Manasseh; 18 and Joseph saith unto his father, `Not so, my father, for this [is] the first-born; set thy right hand on his head.' 19 And his father refuseth, and saith, `I have known, my son, I have known; he also becometh a people, and he also is great, and yet, his young brother is greater than he, and his seed is the fulness of the nations;'
Romans 11 (ESV)
11 So I ask, did they (Judah among the Jews) stumble in order that they might fall? By no means! Rather through their trespass salvation has come to the Gentiles (and Ephraim among them), so as to make Israel jealous. 12 Now if their trespass means riches for the world, and if their failure means riches for the Gentiles, how much more will their full inclusion mean! 13 Now I am speaking to you Gentiles. Inasmuch then as I am an apostle to the Gentiles, I magnify my ministry 25 Lest you be wise in your own sight, I do not want you to be unaware of this mystery, brothers: a partial hardening has come upon Israel, until the fullness of the Gentiles has come in.
Genesis 48 (Young's Literal Translation)
17 And Joseph seeth that his father setteth his right hand on the head of Ephraim, and it is wrong in his eyes, and he supporteth the hand of his father to turn it aside from off the head of Ephraim to the head of Manasseh; 18 and Joseph saith unto his father, `Not so, my father, for this [is] the first-born; set thy right hand on his head.' 19 And his father refuseth, and saith, `I have known, my son, I have known; he also becometh a people, and he also is great, and yet, his young brother is greater than he, and his seed is the fulness of the nations;'
In verse 11 you have added your words to scripture. I will post the context of verse 11 and we will see who "they" are according to the grammar. Romans 11:7-13;
7 "What then? Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded
8 (According as it is written, God hath given them (Israel) the spirit of slumber, eyes that they (Israel) should not see, and ears that they (Israel) should not hear unto this day.
9 And David saith, Let their (Israel's) table be made a snare, and a trap, and a stumblingblock, and a recompence unto them (Israel):
10 Let their (Israel's) eyes be darkened, that they (Israel) may not see, and bow down their (Israel's) back alway.
11 I say then, Have they (Israel) stumbled that they should fall? God forbid: but rather through their (Israel's) fall salvation is come unto the Gentiles, for to provoke them (Israel) to jealousy.
12 Now if the fall of them (Israel) be the riches of the world, and the diminishing of them (Israel) the riches of the Gentiles; how much more their (Israel's) fulness?
13 For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles, I magnify mine office
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25 For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.
I think that I do not have to say much. "THEY" of verse 11 are not "the Gentiles with Ephraim among them". It is ISRAEL according to the grammar. And by using the word "HE" or "IT" seeketh in verse 7 the Holy Spirit shows us that Israel is all who came from Jacob, not the ten northern Tribes. Then, not only does the grammar imply "Israel" for "THEY" and "THEIR", but Israel is CONTRASTED with the Gentiles. That is, Ephraim is always included in ISRAEL and is in CONTRAST to the Gentiles. Your addition of Ephraim is groundless.
Next, the word "fulness" in verse 12 is the same word as in verse 25. Thus, what ever meaning you give the one, you must give to the other. But even this does not matter because ONE is for Israel, which contains the Tribe of Ephraim, and the OTHER is the GENTILES, who are CONTRASTED. And your insertion of Genesis 48:19 helps nothing. It is written in Hebrew and the word "fulness" in the Hebrew means "to make full", "make replete" or "to fill up as full as the nations are in number". I quote the Cambridge Bible for Schools and Colleges Comments form Biblehub;
a multitude] Lit. fulness, as Isaiah 31:4, “a multitude (lit. fulness) of shepherds.” To become “the fulness of the nations” is to be as full of population as all the nations of the world; a strong hyperbole.