How does "fellow heirs" in Romans 8:17 relate to "those who are to inherit salvation" in Hebrews 1:14? i.e. If the children are fellow heirs with Christ and the children inherit salvation, does that mean that Christ also inherits salvation?
That is a very perceptive question. The answer is YES. Consider Hebrews 5:7
7 In the days of his flesh, Jesus offered up prayers and supplications, with loud cries and tears, to him who was able to SAVE him from death, and he was heard because of his reverence.
Christ needed to be saved. Not from moral sin, but from death. Often we think of the salvation that Jesus offers as exclusively a moral salvation from our very sins, but the Jesus solution is also a fix to a very practical problem, how can we live with a holy God, the pure source of life and excellence, when our entire existence is plagued with weakness, physical failure (physical sin), and death. As it is written,
50 I tell you this, brothers: flesh and blood cannot inherit the kingdom of God, nor does the perishable inherit the imperishable... 53 For this perishable body MUST put on the imperishable, and this mortal body MUST put on immortality. 54 When the perishable puts on the imperishable, and the mortal puts on immortality, then shall come to pass the saying that is written:
“Death is swallowed up in victory.”
55 “O death, where is your victory?
O death, where is your sting?”
56 The sting of death is sin, and the power of sin is the law. 57 But thanks be to God, who gives us the victory through our Lord Jesus Christ. 1 Corinthians 15:50,53-57
We participate with Jesus in this victory by participating in his death through which both he was saved and by which he does save. This union with Christ's death occurs scripturally through baptism:
3 Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death?
4 We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life. 5 For if we have been united with him in a death like his, we shall certainly be united with him in a resurrection like his. Rom 6:3-5
The absolute significance of Christ death is why Christians should regularly proclaim his death through the participation (communion):
26 For as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes. -1 Corinthians 11:26
It is our access, through Christ's death, to Christ's salvation (the resurrection) that we are saved:
20 ...the Lord Jesus Christ, 21 ...will transform our lowly body to be like his glorious body, by the power that enables him even to subject all things to himself. -Phil 3:21
10 For if while we were enemies we were reconciled to God by the death of his Son, much more, now that we are reconciled, shall we be SAVED BY HIS LIFE.
When as "fellow heirs with Christ" is the Christian's inheritance the same as firstborn Jesus' inheritance...
Well, as we discussed we share in the inheritance of the resurrection (Christ's eternal body/the new creation), but the idea of us being co-heirs is greater than this for we will also inherit his kingdom.
34 Then the King will say to those on his right, ‘Come, you who are blessed by my Father, inherit the kingdom prepared for you from the foundation of the world. -Matthew 25:34
But this inheritance won't merely be ours as mere citizens, but it will be ours to rule over alongside the Anointed.
11 The saying is trustworthy, for:
If we have died with him,
we will also live with him;
12 if we endure,
we will also REIGN with him;
21 The one who conquers, I will grant him to sit with me on my throne, as I also conquered and sat down with my Father on his throne. 22 He who has an ear, let him hear what the Spirit says to the churches.’” Revelation 3:21-22
18 But the saints of the Most High shall receive the kingdom, and possess the kingdom for ever, even for ever and ever.’...
27 And the kingdom and the dominion, and the greatness of the kingdoms under the whole heaven, shall be given to the people of the saints of the Most High; their kingdom is an everlasting kingdom, and all dominions shall serve and obey them.' -Daniel 7:18,27 (JPS Tanakh 1917)
...and when different?
Well, why we are members of the church (which if we take seriously what the Bible has to say, will ultimately amount to the assembly - a ruling class of divine beings), Jesus is its head:
And he is the head of the body, the church. He is the beginning, the firstborn from the dead, that in everything he might be preeminent. -Colossians 1:18
And while we are a royal priesthood, he is the High Priest:
17 Therefore he had to be made like his brothers in every respect, so that he might become a merciful and faithful high priest in the service of God, to make propitiation for the sins of the people. -Hebrews 2:17
And while we shall be rulers over God's kingdom, Christ is the King of Kings:
14 They will make war on the Lamb, and the Lamb will conquer them, for he is Lord of lords and King of kings, and those with him are called and chosen and faithful.” -Revelation 17:14
[Is Deuteronomy 21:16-17 relevant?]
Kinda, Jesus's portion is essentially everything...
1 Corinthians 15:27 27 For “God has put all things in subjection under his feet.” But when it says, “all things are put in subjection,” it is plain that he is excepted who put all things in subjection under him.
...while it seems we, individually, will have some divided-up jurisdiction...
Luke 19:17 17 And he said to him, ‘Well done, good servant! Because you have been faithful in a very little, you shall have authority over ten cities.’
...So... It seems like Jesus gets a little more than a double the portion.