My understanding of the RPW is that it regulates more than just the element (depending on how the term element is applied). We are not just to sing but to sing psalms (even in the non-EP sense the biblical mandate is still quite narrow - the idea only the element of singing is commanded seems to nulify any discussion of what is to be sung: Exclusively Psalms, Inclusively Palms, Excluding Psalms). Etc. There is a good deal of direction given on the substance of the element. With prayer isn't the normative biblical example one person praying publicly? Even in the directory it says the minister should pray, I don't see anything about a corporate confession (where every speaks/reads a prayer at once). Not saying this proves it but it doesn't seem to be in mind.
Yes, the element is regulated by the general rules of the word. Even in reformed congregations that don't practice EP, the singing is regulated by the commands related to it. They must wisely teach the Word, not something else. The same would be true of prayer. If there is a concept of praying together in Scripture, then there must be a way to apply it. Yes, most often we see one person leading it. That doesn't rule out the possibility or usefulness of a corporate prayer. Again, in a corporate setting, we are not just worshipping, we are also teaching (hence the common practice of corporately praying the Lord's Prayer). And in reformed history, even the structure of worship is often shaped by our covenantal understanding of God, and how we relate to him. We are not required to structure our liturgy this way, but it is useful for edification and reinforces our theology. A corporate confession can have a role within that, teaching and reinforcing how to pray consistently with our understanding of God. It's not required, but it could be wise or edifying to do it.
This isn't clicking for me. David's confession was recorded with inspiration as a Psalm which we are commanded to sing - different element. I do not question that Psalm 51 (or the others) should be sung together - I think it should be done & often. It seems the case for dance to be a part of displaying joy in worship with this approach rather than as a specific element (not saying I agree just trying to logically follow this out). Although I agree the Psalms are useful for instruction the use of the Psalm as a public confession is itself commanded when sung. It seems a bit beyond the intent to say we should borrow from one element to determine another. The psalms speak of many things that we are blessed to sing of but would never incorporate into another element. Their blessing is in the sung psalm.
The Psalms are not just songs to be sung, but Scripture and prayers as well. Scripture itself is clear that the Psalter is much more than a hymnbook. As Scripture, they are God's revealed Word to be preached and read, not just sung. As inspired prayers, they teach us how to pray as well, which includes a corporate dimension. The full content of the Psalms is given for all these purposes, not just for singing. Even if you believe in EP, you cannot reduce the use of the Psalter to just a hymnbook or the content of song.
We visited two OPC churches. One follows a more traditional liturgy (Cavin's) and the other I'm not sure if they follow a liturgy but they have rotating worship style for singing that always includes some form of on stage "worship band" and may or may not include psalms. This doesn't leave much difference between "reformed worship" and other "evangelical worship" styles. This less strict OPC church we visited didn't seem very different from our current non-denominational or previous efree churches.
Just as a point of conversation the OPC with the worship band (not sure what else to call it) was the first time my wife and I had been to an actual Presbyterian church. From our studies of the WCF & related documents and affirming them as biblical we left very confused on how that was reconciled to the confession. They used acoustic instead of electric instruments and a hand drum instead of a kit but that is just preference of modernity vs traditionalism - in principal the worship wasn't really any different from our non-confessional church. The second OPC we visited had no such "worship band". In your understanding of the RPW there is room for either style without conflict? I am struggling to see how anything is regulated in this view. Most churches I have been in are not inventing new things like a part of service that is dance, band performance or smoke machines but rather would be changing what I gather you are calling circumstances.
No, the RPW does not address musical style or instruments as an element, not directly anyway. Yes they would fit under circumstances or forms and are matters covered under the general rules of the Word and wisdom. That does make it more difficult to work through those questions, but not impossible. Is it wise to use a worship "band"? Does it interfere or promote congregational singing? Does it drown out the human voice, or merely guide it? Musical tunes also communicate messages. Do they fit with the words and meaning of the text? These are all matters covered by general rules of the Word and wisdom in the local setting. And that is actually part of the beauty of the RPW, which makes it useful to transcend different congregations or cultures and the way they express devotion to God. I've seen worship "bands" used in a beneficial way and in a disruptive and distracting way. And I've also had the same experience with an organ or piano. And often, traditional or contemporary music is used to promote an unbiblical emphasis on a type of experience in worship, a defective theology of worship. But this would fall under the general teaching of the Word and wisdom to correct that.
And yes, there is often a difference between a typical "reformed" vs. "evangelical" order of service. But such differences in liturgy are allowable under the RPW. We are not given a liturgy. How you arrange the elements is flexible. And yet, some could arrange the elements to promote a defective theology of worship, (i.e. the over-emphasis on experience). The problem to confront there is the defective theology behind the liturgy, not the elements themselves (unless of course an element like a song or sermon is also teaching that defective theology). But still, you must be careful to investigate before judging that liturgy, especially if you are comparing liturgy cross-culturally or in a congregation in the process of reform.
What types of instruments to use today vs. others is a question for prudence since Scripture doesn't address it. Even if you argue, as some do, that musical instruments are elements that disappeared with the Temple, you are still left with questions of musical style in the tunes you choose and the way in which you sing (i.e. only Western tunes? unison or parts? shall we not sing harmony because Scripture doesn't command it?, etc.), and it's still a question of wisdom. Is it orderly, is it beneficial, does it help communicate the Word which is sung? Etc.
Even in an EP congregation, you are singing pre-determined arrangements of words, tunes, and even vocal parts. And how is that different from a corporate prayer written by the minister? Both are led activities directing their hearts to God in a way consistent with the broader teaching about the element, and all within the RPW.
I agree but is the activity of everyone praying a confession together in worship commanded or is it the minister praying? In my mind this is a pretty big difference.
I guess I don't see it as a "big" difference so long as you are following the general rules of the Word with wisdom. We do it with singing, as noted above. And what about a corporate reading or responsive reading of Scripture? Some psalms are written in an antiphonal pattern, apparently intended to be read back and forth between leader and congregation or between two groups. In Deuteronomy the congregation reads out the blessings and curses. Is that the usual way? No. Ordinarily, it's one person reading, and that's usually the wisest way to do it. But there are times it could be wise or useful to do it corporately and still allowable by the RPW.
Hopefully, that is helpful. If I'm still not getting to your question feel free to prod more.