I just read your first post, so forgive me if you addressed this later on in the thread. I am not convinced that you are arguing from a consistent regulative principle framework. For instance, you state:
"The fact is there is not a command to sing the book of Psalms exclusively." No one, and I mean no one who understands and defends the Presbyterian & Puritan view of worship, believes the Bible commands the exclusive use of Psalms as a positive command. In other words, we do not go to the scripture to see commands such as "you shall only do X." Rather, we see the command to sing Psalms and no clear and unambiguous reason to sing anything else.
You also state:
The EP advocates try to make that case by arguing that "psalms, hymns, and spiritual songs" in (Col. 3:16, Eph 5:19) refers the "book of Psalms". Those arguing for inclusive psalmody (singing Psalms but not exclusively) argue that there is no evidence within these texts to deduce Paul meant "the Book of Psalms only".
This is also a misunderstanding of our position. We do not argue positively from Col. 3 & Eph. 5 as proof for exclusive psalmody divorced from the canonical context. Rather, we see these three phrases used in the psalms themselves so there is no definitive proof that any of these three terms
must mean an uninspired hymn.
Also:
The inclusive psalmody advocates are not persuaded that the argument for EP can be deduced by good and "necessary" consequence, either by what is commanded, or by the natural use of grammar or context into which Paul spoke there.
The burden of proof is not on the Psalm singer to prove the exclusivity of the Psalter. The burden of proof is on the Hymn singer to prove that singing uninspired hymns is necessitated by the NT text.
The boundaries for the content of our song is "the Word"
Amen. Carry this out consistently. The content of our song is limited by our song leader. In Christ, we sing with Christ and to Christ. He leads his people with his praise to the glory of his own name. The Word of Christ in this life is nothing other than the inspired praises of scripture.
Another unhelpful argument is trying to make "singing" an entirely separate category of worship.
Can a congregation be faithful to the commands of scripture if they never sing at all? If not, then it is admitted that sung praise is a separate element and must be regulated and defended on its own. Surely there is overlap in all of the means of grace, but the overlap does not overshadow the distinctions. Women are to sing psalms in the midst of the congregation, but that does not mean women are to pray on behalf of the congregation.