"worldly" (literally "fleshly") and "unspiritual" Christian: one cannot be a Christian and be lack of the Holy Spirit but they can think and behave just like the people without the Spirit.
Paul describes the Corinthians as mere infants in a pejorative sense. The argument of 2:6-16 implies that for Paul the gospel of the crucified Christ is both "milk" and "solid food". The Corinthians do not need a change in diet but a change in attitude and perspective.
The fact that leaders are servants only stands as the divine contradiction to a merely human understanding of the role of leaders. Even Jesus was among men as one who served (Lk 22:27).
"it" = the church as a whole, not individuals; similarly "you" [plural] in v.9,16,17 referring to the church as a whole (contrasting the use of "temple" in 6:19). Apollos and Paul are servants only; God is at once both responsible for growth (v.6-7) and the owner of the field (v.9).
Both Paul and Apollos are nothing ("neither...is anything"). They have essential tasks to perform. Yet, from the perspective of ultimate responsibility for the Corinthians' existence as the people of God, Paul and Apollos count for nothing. God alone saves and sanctifies.
Unity and diversity of the farm labourers -- their aim is one, a rich harvest; but they have different tasks of planting and watering. So are the church leader-servants.
The list of 6 materials is indeed a scale of descending value. "Gold, silver, and costly stones" represent what is compatible with the foundation (Christ). They recur in the OT to describe the building materials of the Temple (1Ch 22:14,16, 29:2, 2Ch 3:6), thus leading to the imagery of v.16-17. The choice is between building with imperishable materials (gold, silver and precious stones) of the gospel or perishable materials (wood, hay or straw) of human wisdom.
Those who are building with "wood, hay, or straw" will suffer loss of their work, not loss of their salvation. The precise nature of the reward or the loss is not known. It is the work that is consumed, not the labourer himself/herself. It is a warning--and encouragement--to those responsible for "building" the church and those with teaching/leadership responsibilities.
Paul's combative style is again illustrated by "Don't you know that..." which is used 10 times in this letter, but only one time in his other letters (Ro 6:16); compared to the more gentle way he reminds the Thessalonians: "For even as you yourselves know..."
Paradox in the gospel contrary to human understanding: wisdom is folly; folly is wisdom; weakness is power; leaders are servants; God's people are nobodies, yet possess all things (v.22- 23, 2Co 6:9-10). God's people must abandon confidence in the securities of the present age; they must trust in God's folly--"he should become a fool"--and thereby become truly wise.
They may not say "I belong to Paul, or Apollos, or Cephas" because the reality is the precise opposite, that is, they all belong to you. The untranslated "for" connects the two phrases.
List of Ďall things': world, life, death, the present, and the future. These are the ultimate tyrannies of human existence, to which people are in lifelong bondage as slaves are: (1) the physical world, (2) the corruptible life, and (3) the unstoppable flow of time. Now, in Christ, they are free lords of all things and no longer under its bondage.