Part 1 is available here.
TWH: The participle in question is unique in the New Testament. The adjective is found in the LXX translation of Lev. 19:19b, which reads, "You shall not breed two different kinds of cattle." The use of the word in 2 Cor. 6:14 shares the sentiment found in the Leviticus passage. It is very likely the author has this specific passage in mind when he uses this word. The same idea is found in Deut. 22:10 as well.
I like Antonio's translation of 2 Cor. 6:14 a lot: "Do not yoke yourselves together with unbelievers." A case for "stop" in this context could be made, i.e., "Stop yoking yourselves together with unbelievers."
Regarding whether 2 Cor. 6:14–7:1 is a non-Pauline interpolation, much can be said. In fact, much has already been said. Every commentary touches on this issue. There are journal articles galore on this very topic. While it's true that many people do not believe this pericope is (1) original to the this letter and (2) written by the apostle Paul, there are a number of scholars who have and do maintain that the verses are Pauline and were originally written in the place where they are now found, between 2 Cor. 6:13 and 7:2. (One reason for the larger portion of the scholarly population being in favor of a non-Pauline interpolation, I presume, is there are vastly more scholars of the New Testament who are not "confessional.") G. Guthrie, for example, talks about the "balance" exhibited in these verses and the larger discourse section in which they are found. Paul used digressions regularly in his writings. If you think about it, we all do when we write (or, nowadays, when we type). And digressions do not have to be purely uncontrolled spontaneous outbursts or completely disconnected discourse gibberish. Paul makes sense here. He wants believers to be sure that they are not aligning themselves in binding relationships that will pull them into idolatrous decision-making, and thus not honor the Lord with their lives and possessions and forfeit the type of testimony they should have as light among darkness. Murray Harris summed the flow of the argument up pretty well in his commentary:
"Paul is indicating why the Corinthians are restricted in their affections for him (6:12) and how they can enlarge their hearts toward him (6:13). They are continuing to flirt with paganism, but now must fully comply with his earlier injunctions to shun idolatry in any form . . . and to shun all immorality . . . . Openheartedness to Paul and full reconciliation with him would be achieved only when they made a total break with paganism. Such a break would demonstrate their reconciliation to God (5:20) and their ongoing receipt of God’s grace with benefit (cf. 6:1)" (The Second Epistle to the Corinthians, 497).Likewise, Matera writes, "[H]e is saying that although they may associate with unbelievers in the ordinary affairs of life, they must maintain boundaries between themselves and unbelievers in those things that lead to idolatry" (Matera, II Corinthians, 162).
While this verse might be used by some to suggest that believers should not wed unbelievers, the verse should hardly be limited to such unions.
Just one quick final thought. Occasionally you might hear someone talk about the heavy concentration in this discourse unit of words not used elsewhere in the New Testament. Such arguments shouldn't really convince us, especially when it comes to whether this passage is original or not to the apostle Paul. (It also does not convince me when people use this argument for why Paul must not be the author of Hebrews, but that's another day.) It is true that there are a handful of words in this discourse unit that are not used elsewhere. But the word whose meaning we were discussing initially in the post, we know, is not foreign to the apostle Paul for two reasons:
(1) Paul uses a similar word in Phil. 4:3 (σύζυγος); for him to use the compound form we find in 2 Cor. 6:14 is not impossible to imagine. Paul was the king of compounds. He does so with ἑτερο- in an interesting way in 1 Tim. 1:3 and 6:3. And there's always the Corinthian examples of ἕτερος in 1 Cor. 14:21 and 2 Cor. 11:4 (as well as 2 Cor. 6:14; see also Gal. 1:6), though not compounded.
(2) Paul knew the Old Testament better than perhaps anyone alive today. And you cannot take the Old Testament out of this man. He can pull together an Old Testament reference without even thinking about it, because it is embedded in his persona. Though there is no other record of this verb used in the rest of the New Testament, it's not impossible to imagine that Jews used this word regularly in their interactions and teaching.