I'm finally fully operational on my Mac, and I've uncovered my old email and password I use for my posting abilities on this site. And as I logged in I was hit with your post: Should a Christian Sue a Christian?
Your biblical analysis is spot on, of course. And I don't really see how anyone could argue with what you've written. I'd just like to add a couple points to complement your post.
The question you presented had to do with a lease agreement. If the offending party to the lease is a churchgoing Christian who refuses both to pay the lease and to leave the premises. In that circumstance, I suppose you would go to question (3) in your post, and perhaps conclude that person is certainly exhibiting no signs of being a believer, since his actions force the premises owner to pay, for instance, a $600.00 per month mortgage to let the erstwhile renter squat indefinitely. In that circumstance, if the premises owner were ultimately unable to pay, the bank would foreclose and ultimately forcibly remove the squatter, who now is in a worse position than he was in before.
I'm a litigator by trade, and most of the cases I handle are in the context of personal injury defense. Insurance companies hire me to represent their insureds in cases filed by injured parties. With the prevalence of insurance, believers don't really have the opportunity to work out their conflict. If I'm in a car wreck with a member of my church, and we both agree it's his fault and he should be responsible for my damages, we can't force the insurance company to pay what we both think would be a reasonable sum. (There are good reasons for this, of course.) In that circumstance, I think we're outside of what Paul was contemplating, since both sides to the conflict are in basic agreement, they're just trying to get a third-party corporation to pay what the offending party would owe.
But getting back to the examples you were dealing with: One issue that comes to mind immediately is a lament of the loss of the parish church. Until quite recently in history your neighborhood would all attend the same church. You would be in commerce with your neighbors, you'd worship with your neighbors, and you would see them often during the week. There would be real relationships there. Now, church members see their fellow church members on Sundays only, as a rule. And their pastor has little authority over them since they can bolt for the next church, which will likely have a "no questions asked" policy regarding why you left your last church. (And if they do ask questions, they'll only ask you, and won't get both sides of the story.)
A few years ago my wife and I went through a course on biblical peacemaking; it was created by a lawyer named Ken Sande, who is, incidentally, a Calvinist, after he wrote a book called "The Peacemaker: A Biblical Guide to Resolving Personal Conflict." His goal was to equip Christians with the principles of biblical peacemaking, with a view toward restoration of relationships. I still remember the "four g's of biblical peacemaking": Glorify God; Get the log out of your own eye; Gently restore; Go and be reconciled. In that course, we were encouraged to let slide any offense against us that we could let go. If some offense was too great to let slide, we were encouraged to speak with the other person gently, in person, always keeping in mind that the goal is to bring glory to God. We were taught that getting the log out of your own eye is critical in these conversations, but so is pointing out the offense. I've utilized these principles in my personal life and they're well worth considering.