I understand Paul gave good instruction and advice concerning specific "things whereof ye wrote unto me" (1 Corinthians 7:1).
What specific thing was Paul addressing when he wrote:
1 Corinthians 7:6-9 But I speak this by permission, and not of commandment. For I would that all men were even as I myself. But every man hath his proper gift of God, one after this manner, and another after that. I say therefore to the unmarried and widows, It is good for them if they abide even as I. But if they cannot contain, let them marry: for it is better to marry than to burn.
or when he wrote:
1 Corinthians 7:12-16 But to the rest speak I, not the Lord: If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away. And the woman which hath an husband that believeth not, and if he be pleased to dwell with her, let her not leave him. For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy. But if the unbelieving depart, let him depart. A brother or a sister is not under bondage in such cases: but God hath called us to peace. For what knowest thou, O wife, whether thou shalt save thy husband? or how knowest thou, O man, whether thou shalt save thy wife?
or when he wrote:
1 Corinthians 7:25-28 Now concerning virgins I have no commandment of the Lord: yet I give my judgment, as one that hath obtained mercy of the Lord to be faithful. I suppose therefore that this is good for the present distress, I say, that it is good for a man so to be. Art thou bound unto a wife? seek not to be loosed. Art thou loosed from a wife? seek not a wife. But and if thou marry, thou hast not sinned; and if a virgin marry, she hath not sinned. Nevertheless such shall have trouble in the flesh: but I spare you.
any answer other than "I don't know" is pure conjecture. All we know is what Paul wrote in response to specific issues the church in Corinth had requested Paul address.
I understand Paul was responding to specific matters the Corinthians had written about (1 Cor. 7:1). But what, specifically, were those matters?
For example, what was the question behind 7:6–9? Or 7:12–16? Or 7:25–28? Paul never tells us what the Corinthians originally asked. We only have his answers.
So when someone claims to know precisely what issue prompted a particular statement, they need to demonstrate it from the text. Otherwise, they are reconstructing the Corinthians’ questions from Paul's answers.
We can certainly infer possibilities, but we cannot present those reconstructions as certain. Beyond what Paul actually tells us, “I don't know” is sometimes the most honest answer.
We still do not know what issues the church had asked Paul to address.
I believe Paul was faithful in providing sound guidance and counsel concerning the issues about which the church had written in the letter to Paul.
Paul walked in wisdom in indicating what he spoke by commandment and what was his own opinion concerning specific issues the church wrote to him about, the specific situational questions which we are not privy to.
But that is precisely the distinction that matters: we may not have the Corinthians' original letter, but we do have Paul's answers to the issues he says they raised.
Paul explicitly identifies subjects he is addressing: marriage and sexual relations (7:1–5), the unmarried and widows (7:6–9), separation from a spouse (7:10–16), and virgins and marriage (7:25–28). We don't need to reconstruct the original letter word-for-word to know what subjects Paul is discussing.
Likewise, “not the Lord” does not mean “merely my opinion.” In 7:12 Paul distinguishes his instruction from a command previously given by Jesus; he does not thereby disclaim authority for what follows.
So yes, there are details about the Corinthians' original questions that we cannot recover. But saying “we don't know the exact questions” is not the same as saying “we cannot know what issues Paul was addressing.” The latter goes beyond what the text warrants.