Catholics definitely maintain that the Bible is not always easy to understand. It makes sense, if you remember that it was written by people in a different time period, in a different part of the world, in a different culture, and that their audience was people in that time period and region and culture. Even translations of secular modern literature often have footnotes and study guides to go along with them.
I agree, and the footnotes, and study guides need to make sense, otherwise they are put to question. For instance, you say that Paul was refuting the Corinthians, yet there is nothing in the passage itself to suggest he is refuting their claim. And the commentaries well they don’t shed any light, on explaining step-by-step how paul’s comment is meant to refute.
Instead of explaining the verses, the commentary of this verse is a diatribe about good men.
Matthew Henry’s Concise Commentary
12:11-21 We owe it to good men, to stand up in the defence of their reputation; and we are under special obligations to those from whom we have received benefit, especially spiritual benefit, to own them as instruments in God’s hand of good to us. Here is an account of the apostle’s behaviour and kind intentions; in which see the character of a faithful minister of the gospel. This was his great aim and design, to do good. Here are noticed several sins commonly found among professors of religion. Falls and misdeeds are humbling to a minister; and God sometimes takes this way to humble those who might be tempted to be lifted up. These vast verses show to what excesses the false teachers had drawn aside their deluded followers. How grievous it is that such evils should be found among professors of the gospel! Yet thus it is, and has been too often, and it was so even in the days of the apostles.
The commentary below makes a blanket statement “is hereby anticipated and refuted”, it is a presumption of the commentator rather than an explanation.
Jamieson-Fausset-Brown Bible Commentary
16. I did not burden you-The “I” in the Greek is emphatic. A possible insinuation of the Corinthians is hereby anticipated and refuted: “But, you may say, granted that I did not burden you myself; nevertheless, being crafty, I caught you (in my net) with guile”; namely, made a gain of you by means of others (1Th 2:3).