First, can we really say that the Jerusalem church is a model for all to follow? James ended up taking the role of leadership of the church, but he certainly did not act in isolation, as even your friends admit. But notice Acts 15 first, then Galatians 2. In Acts 15.6, we are told that the apostles and elders consulted together to decide on the issue of the necessity of circumcision. Peter speaks first, giving his testimony. This is followed by a report from Paul and Barnabas. Then, in v. 13, James speaks. But he starts by saying, "Brothers, listen to me." That could possibly indicate that he is taking charge, but it could also--and in this context, I think it likely--be that he is simply weighing in with his opinion. In v. 19 he says "It is my judgment" or "I conclude." This is the word krino, as you noted, but it is a far cry from seeing in this word that James is exercising an authority above the rest in this matter! The same word is used in several other passages that indicate nothing of the sort. For example, note Acts 13.46 ("you do not judge yourselves worthy"); Acts 26.8; Rom 3.4 ("You will prevail when you [God!] are judged"); Rom 14.3-4 (various believers judge each other); Rom 14.5 (one person judges one day to be holier than another); cf. also Rom 14.10, 13, 22; 1 Cor 2.2 (referring to a personal commitment or decision); 1 Cor 10.15 ("you judge what I say"--Is Paul saying that the Corinthians are authorities above him? Hardly); etc.
The point is that this word is used in a variety of ways, and one ought not to import into the text of Acts 15.19 what the context says is not there. And the context of Acts 15 clearly says that James did not act alone. In fact, it is hard to see him as doing anything other than expressing the will of the whole group. See v. 22 (the apostles and the elders decided); v. 23 (the letter is explicitly from the apostles and elders); v. 28 ("It seemed best to the Holy Spirit and to us" surely indicates that the group decision was in reliance on the Spirit's guidance, not James' guidance). The whole chapter indicates that a group decision was made, with input from Peter, Paul, Barnabas, and James. The whole group decided, not James. Further, and this is most important, the group was not just elders. It was apostles and elders. Surely that indicates that the Jerusalem church was different from any other church that was to follow and ought not be a model for us in every respect--unless, of course, we believe in apostolic succession!
Now, when one compares this to Galatians 2, a different picture emerges. Most scholars believe that Gal 2 is not talking about Acts 15, but is referring to an earlier, non-official meeting that Paul had had with leaders of the Jerusalem church. If James was the head honcho, why doesn't he get more respect from Paul? But notice what Paul says. Read Gal 2.4-9 sometime. It is obvious that Paul does not think that James has any authority over him in the least! Rather, as an apostle he was guided by the Spirit of God, and he would not allow anyone to change what the gospel was. Further, one might argue, on almost as good a basis, that Peter had every bit as much authority as James in the early church. He certainly was the first to be given the mandate from the Lord (Matt 16). In the least, if there was a pecking order, Peter would certainly not be lower in the chain than Paul. Yet, Paul goes on to rebuke Peter for forsaking the implications of the gospel! As well, Paul and James seem to be in conflict elsewhere in the NT: Jas 2.14-26 vs. Rom 3.21-31. This was such a conflict that Luther declared, because of this passage, that James was an epistle of straw! I think that he misunderstood James here, as many have, but he understood Paul correctly. In the least, whatever else this shows it certainly shows that Paul did not regard James as an authority over him. And that is the point: Even if James were considered to be the leader of the Jerusalem church (a view which I adopt, by the way), the question of whether this is a model for us today and what kind of authority he had over his fellow apostles and elders remains to ber answered. I think the only satisfactory answer is to regard the Jerusalem church, in this respect as well as many others, as NOT a model for us today, and to regard James' role as only slightly more above his fellow apostles. Further, he never seems to have acted unilaterally on behalf of policy making, but always wrestled with and represented the views of the rest of the apostles.
Now, in terms of application to the modern church: I do believe that a single leader needs to emerge from among equals. The Latins called this Primus inter Pares--first among equals. Someone takes the lead and is the spokesman for the group. This is only natural. But he is not given extra powers, does not have authority to act unilaterally, and certainly must work in unison with the rest of the elders. If we see this in the life of James and wish to apply it, so be it. But even that is probably too much, as I said above. I'm simply saying that someone usually acts as spokesman for a group and gets the ball rolling. In that respect, he is primus inter pares. But he's no monarch. Plurality of elders is every bit the authoritative group, not the head elder.