Allow me first to direct you once again to the synopsis of the chapter, and it may also make sense to read parts one and two if you haven't already.
We are now discussing The place of the doctrine in dogmatics. As an aside, I think Barth is the theologian with the greatest appreciation for what might be termed the architecture of theology; for him, order of presentation matters, and should reflect the internal order of God's self-revelation. So, the question that is answered by this sub-section is: why put the doctrine of election here, within the doctrine of God, ahead of discussion of Christology proper, ahead of creation, ahead of sin and reconciliation? "As far as I know", Barth admits, "no previous dogmatician has adopted such a course" (76). So it requires some justification.
The straightforward answer is given at once: Barth wishes "to maintain of God that in Himself, in the primal and basic decision in which He wills to be and actually is God, in the mystery of what takes place within Himself, within His triune being, God is none other than the One who in his Son or Word elects Himself, and in and with Himself elects His people" (76). Election, for Barth, is God's own self-definition of himself, as the one who wills not to be without his people. "There is no height or depth in which God can be God in any other way" (77). Therefore, the doctrine of election belongs to the doctrine of God, and the latter is not complete (indeed, is fundamentally deficient) without it. This is controversial, and I'll say more about it at the end.
By way of explaining his innovation here, Barth offers six alternative locations in which the doctrine of election has been placed historically (although the last three are pretty similar and treated fairly briefly). In summary, they are as follows:
1. The position of 17th century Reformed Orthodoxy, in which "the doctrine of predestination followed closely upon the doctrine of God, preceding directly the doctrine of creation" (77). Although Barth wishes to uphold the positive concern of this tradition, he cannot quite adopt it, because as he sees it the Reformed Orthodox did not actually prioritise election, but "the tenet of the decrees of God in general", and so "it takes God in His general relationship with the world as its first datum, and understands His electing as one function in this general relationship" (78). For Barth this is to get things backwards, and undermines the positive intention of the Reformed theologians.
2. There is a minority position which adopts the sequence: doctrine of God; Christology; creation; predestination. The valid insight here is "that the work of God (the work of all works!) is not creation, but that which precedes creation both eternally and in effect temporally, the incarnate Word of God, Christ" (80). (Note that 'in effect temporally' - also controversial!) However, the failure to co-ordinate Christology and election here makes this schema unacceptable to Barth.
3. The third arrangement treats election "quite simply within the context of the doctrine of the Church" (81), i.e. after creation, sin, etc. For Barth this has the virtue of a "direct relationship with the Bible" (83), which does represent God's election in terms of God's people. However, for Barth placing the emphasis so squarely on the elect people rather than the electing God causes difficulties - not least when it comes to assurance (on which more later).
Arrangements 4, 5 and 6 can be grouped together: they place election at the beginning, middle, or end of the doctrine of reconciliation, seeing it as the first, central, or final word in that doctrine. For Barth it doesn't really matter which of these is adopted: if election is first, it must also be central and last, and the same could be said of the other arrangements. The problem here is that all these treatments fail to recognise that "the doctrine of reconciliation is itself the first or last or central word in the whole Christian confession" (88). For Barth "Dogmatics has no more exalted or profound word - essentially, indeed, it has no other word - than this: that God was in Christ reconciling the world unto Himself" (88). How, then, can there be a doctrine of God which is not already essentially about the election which takes place in Christ?
The danger of making the doctrine of election "supplementary and secondary" (89) is very great. Besides the fact that it leaves the doctrine feeling like an afterthought when it is introduced, it has the effect of distorting other doctrines. For example, putting sin before election has the effect that "God Himself appears in a sense to be halted and baffled by sin" (90), sin creating a broken kingdom outside of God's will. This will not do for Barth - "the regnum Christi is not one kingdom with others" (90), but in everything the triune God is sovereign, so that with regard to man Barth can say "Neither in the height of creation nor in the depth of sin is he outside the sphere of the divine decision" (90). In short, all of God's works are to be understood as deriving from his election. "Always and from every point of view they derive from Jesus Christ" (92).
Barth's aim here seems clear: to establish election within the doctrine of God, so that there is no shadow God standing behind Jesus Christ, who may have chosen in one way or another independently of the gospel. The election here is the election of God to be this God, and therefore to be for his people in Christ. We are genuinely prohibited from seeking a view or understanding of election anywhere other than in Christ. Although it's not particularly expanded upon, there is presumably in Barth's mind the knock-on pastoral effect that this has: rather than being advised to ignore the electing God and focus on Christ for assurance (a procedure which is somewhat Wizard of Oz like), we are to focus on the electing God in Christ. There is no other God. I don't think that means that God constitutes himself in this election; rather, God makes himself this God and no other in his eternal election of himself to be the God of Jesus Christ and his people. This is the decision behind which we can never go.