In short, you can't have it both ways. it doesn't make sense to examine similarities between the Book of Mormon text and the text of a book you agree wasn't plagerized, and which was allegedly written on a different subject and written in a different writing style.
Of course it does. The Roman story may not have been plagiarized directly, but it was written by the same author who had been associated by the Con. witnesses with the Book of Mormon. It was not "written on a different subject", but rather a different story on the same general subject and even though it may have been written in a different writing style (on purpose), it was still written by the same author---meaning the author who wrote the Roman story is the same guy who allegedly wrote Manuscript Found. So, yes, it makes a lot of sense to look at his extant manuscript and make comparisons with the Book of Mormon and with Smith's discovery narrative.
This is an excellent point, but unfortunately, it works against your theory in multiple ways. Since the Conneaut witnesses testimonies predate publication of the Book of Mormon discovery narrative, and since the Book of Mormon did not, itself, contain the discovery narrative; and since the witness testimonies mention a discovery narrative ("represented as being found in this town", "recovered from the earth", "...opened a great mound, where there were human bones. There he found a written history...", "dug up out of one of the mounds in the region", "Spaulding's romance professed to find the Record where the Recorder concealed it, in one of those mounds, one of which was but a few rods from Spaulding's residence"), then at least this aspect of the witness testimony is inconsistent with what they had heard about or read regarding the Book of Mormon at the time, though certainly cnsistent with what one may read in the extant Spalding manuscript. In short, the witness testimonies on this point don't fit what was written in the Book of Mormon, but do fit the extant manuscript (which you agree was not plagerized). It, thus, is evidence against the Book of Mormon being plagerized, and evidence in favor of a single "Manuscript Found". Sorry.
I strongly suspect that Manuscript Found had a discovery narrative in it that closely parallels that found in the Roman story. It is that discovery narrative, I believe, that Joseph Smith copied in 1838 with a few of his own changes and additions. If I am right about that, then everything fits very nicely... witness testimony, account in the Roman story, Smith's account, everything. So while I appreciate your sympathy, I don't think I'll be needing it.