Foreordination does not happen retro-casually/retroactively, with the veil the only way one can know it one was foreordained is through revelation (prophecy, patriarchal blessing, or other other blessing). Even in the pre-existence we had agency, and a third of the hosts of heaven choose not to follow God's plan.
IMO this was not necessarily as simple as one may first read, not only was there a great war in Heaven, but that's not how divisions in opinion happen here on earth. There were probably those more committed/faithful and those less.
Those who choose God's plan choose good (see below vs 3) and we were all at the same standing as one another in that sense (see below vs 5). It would make sense that those more faithful were those who were foreordained to more, keeping in mind that foreordination does not guarantee fulfillment here on earth as we all still have agency. So foreordained individuals have to use their agency here on earth to reach that foreordained calling.
If you read from earlier in Alma 13 from verse 3 you'll find the definition is closer to the given definition
3 And this is the manner after which they were ordained—being called and prepared from the foundation of the world according to the foreknowledge of God, on account of their exceeding faith and good works; in the first place being left to choose good or evil; therefore they having chosen good, and exercising exceedingly great faith, are called with a holy calling, yea, with that holy calling which was prepared with, and according to, a preparatory redemption for such.
4 And thus they have been called to this holy calling on account of their faith, while others would reject the Spirit of God on account of the hardness of their hearts and blindness of their minds, while, if it had not been for this they might have had as great privilege as their brethren.
5 Or in fine, in the first place they were on the same standing with their brethren; thus this holy calling being prepared from the foundation of the world for such as would not harden their hearts, being in and through the atonement of the Only Begotten Son, who was prepared
Definition from Gospel Topics:
In the premortal spirit world, God appointed certain spirits to fulfill specific missions during their mortal lives. This is called foreordination. Foreordination does not guarantee that individuals will receive certain callings or responsibilities. Such opportunities come in this life as a result of the righteous exercise of agency, just as foreordination came as a result of righteousness in the premortal existence.
The doctrine of foreordination applies to all members of the Church, not just to the Savior and His prophets. Before the creation of the earth, faithful women were given certain responsibilities and faithful men were foreordained to certain priesthood duties. As people prove themselves worthy, they will be given opportunities to fulfill the assignments they then received.
In regards to the elect D&C 29:7 says who will here His voice.
7 And ye are called to bring to pass the gathering of mine elect; for mine elect hear my voice and harden not their hearts;