Both of these introductions clearly announce the locutor of the text as the Lord. Each of the sections also proceeds in the first person as commands from the Lord.
The section 18: 12-16 continues with the Lord as the speaker, but the locutor shifts in verse 17 to Isaiah (who is also the locutor in 18). This shift in speaker, is, I suggest, the marker for the shift in the text, and this sealing of the prophetic text marks the end not only to the word of the Lord in this section, but to the conceptual "scroll" announced in the first verse.
Can one be so bold as to suggest such a recutting, even though minor? The translation committee for the NIV notes:
"This poetry is normally characterized by parallelism in balanced lines. Most of the poetry in the Bible is in the Old testament, and scholars differ regarding the scansion of Hebrew lines. The translators determined the stanza divisions for the most part by analysis of the subject matter. The stanzas therefore serve as poetic paragraphs." (Preface to the New International Version, p. xi).
The recutting I am suggesting follows such thematic divisions. If there is an underlying scansion in the Hebrew, however, the analysis would be invalid.