Was Paul offering an olive branch after hitting hard in 1 Corinthians 15:10?
This does not seem to be the case to me. I'm prefacing the passages you provided with verse 9 for further context (NAS):
1 Corinthians 15:9-11: "For I am the least of the apostles, and not fit to be called an apostle, because I persecuted the church of God. But by the grace of God I am what I am, and His grace toward me did not prove vain; but I labored even more than all of them, yet not I, but the grace of God with me. Whether then it was I or they, so we preach and so you believed."
Despite Paul's grand humility in verse 9, he does not appear to diminish his significant role as an apostle, but does temper his statement "I labored even more than all of them" with "yet, not I, but the grace of God with me."
Few would doubt Paul's great labors and the successes with which they were met. Even while (justifiably) elevating the nature of his work, he then attributes all to the grace of God. He is a truly remarkable figure in Scripture.
Hope I've understood your question properly.