These need to be defended before we even turn to Scripture because they are assumotions Penal Substitution theorists make when reading the Bible. Christians here are divided not on Scripture but on assumptions.
Just a note of caution here. It isn't really valid that you can bring in an obscure term (at least on a forum of laymen) like "legal humanism" and then complain that unless we can defend your labeling of us as such that we cannot defend our overall position Rather, why don't we get into the subject matter itself and go from there. What you are doing reminds me of an old comedy show I was watching where someone accused the hero of being "obtuse and one dimensional" to which he replied "I'm gonna go look that up. You might be in trouble."
We are like Calvin, all products of our time. So am I and so are you and my understanding is that law is designed to make society survivable , maybe even pleasant in a fallen world filled with sinful people. And we "mess" with people as little as possible. At least that used to be so. You, Jon, are probably close enough to me, age wise to remember that our motto was "don't hassle me". Thus, law is, whenever possible, concerned primarily with wrong concrete actions primarily because in human government we can't properly regulate motives, attitudes and moral proclivities. God's law, as given for instance in the 10 commandments does indeed include motives and attitudes. The demand to honor your parents, worship God, avoid even coveting, not just actual stealing, etc. And that doesn't even begin to discuss Jesus own internalization of the law in the Beatitudes. In our modern era we experiment with these aspects. Think of laws against adultery, sodomy, hate crimes, prohibitions on alcohol or drugs or pornography and so on, with varied amounts of success. Again, we don't have modern consensus on this and it looks like Calvin's day was no different, but there is no reason we can't go on and discuss atonement without resolving the merits of legal humanism.
Church literature and confessions discuss sin as "doing that which we should not
as well as neglecting to do what we should". And, this is how this relates to our discussion here. The Reformed Calvinists were well aware of all this and so indeed had this whole range of sin in mind in all discussion of atonement and what exactly constituted sin. In addition, much of their literature involves self examination and the necessity for pursuing holiness and the good of others, not just refraining from law breaking. And in addition to that, there is the whole area of our starting out as lost, and enemies of God, whether federal or hereditary and how all that involves the Atonement. We have hardly touched that, but and this is important, that does not mean the Calvinist Reformers were not aware and had no theology on those areas. And, by the way, I don't agree with all their theology in those areas.
Now, Jon, you do have a point in that sometimes in debates, especially with other believers of penal substitution who believed the atonement was universal, where they do frame their arguments using sins as being "things" with an almost physical nature of their own, and drove their points home by illustrations of an economic nature - and that may be a cause for some objections, especially if in their zeal they indeed insist that the whole of the atonement is that and only that. But it really isn't fair to use arguments with other believers in penal substitution, where the starting point was a shared belief in penal substitution, as ammunition to refute what both groups believed in common.
I have read enough at this point to remind us all of a couple of things. One is that the Reformers, Calvinists specifically since they get blamed primarily for penal substitution, did indeed believe and treasure other aspects of the atonement. I can give you references if you need them but we have read many of the same things. What's important from this is to understand that their almost complete emphasis on one aspect of the atonement was I think two fold. One, they were in a real battle with the Arminians, and remember this was winner take all in that church leadership was also running civil society in many areas of life. If you don't believe me just recall Owen's writings in battling the Arminians and you can see his concern for the future of civil society as a whole as well as the theological battle. And two, they were battling Catholic theology, which when you really look into it, is formidable and they had to emphasize that you were not left with the need for the Church or the priest to absolve you of sin and assign penance. This is very similar to the early church in that they were mainly involved with showing that Jesus was actually God and that even though it did not look like it by the numbers, Jesus was victorious and we were winning. And their writing reflects that, even to the neglect of other areas, at least from what we have discovered so far.
In light of that, continue. I learn things from these discussions and don't really mind them if the rhetoric can be kept somewhat civil.