2. See on 7:1 (cf. v. 1 above).
The Western witnesses (D* F G a Ambst) and 
* add δήλω ὑμῖν ὅτι (= “I make known to you that”). One could make good sense of this as the original text, since Paul would thus be protesting his own innocence in the failure of Apollos to come. But if original, it is difficult to account for its deliberate omission in the rest of the textual tradition. Most likely, therefore, it is an attempt to improve on Paul’s sentence, which is a bit awkward due to the opening περὶ δέ phrase.
See on 7:1 (cf. v. 1 above).
Lit. “Apollos the brother.” On the absolute use of “brother” for Paul’s companions, see on 1:1.
Gk. παρεκάλεσα (see on v. 15); cf. the usage in 1:10; 4:13, 16.
As several have pointed out (e.g., Conzelmann, 297), both this unusual past tense and the lack of a greeting from Apollos in 1:1 or 16:20 suggest that he may not be present at the time of writing. Was he urged to go to Corinth, but then moved on to another area of ministry in the meantime?
The strongest argument in its favor is also the strongest argument for Apollos, the absolute use of the word without a qualifier. Cf. Rom. 2:18. Those who take this position include Moffatt, 185; Héring, 185; Ruef, 184; Barrett, 391; Bruce, 160; G. Schrenk, TDNT 3:59; among the English versions, NRSV, ESV, NAB.