The I-IV-I "vamping" is hardly that: It comprises only two measures and is the harmonic underpinning of the the first two measures of the tune (in three of the four couplets). The introduction itself (12 measures) "draws from the opening two lines of the tune." A "more sturdy" texture for the introduction does not seem to be called for, as anything more would detract from the simplicity of the straightforward melodic presentation of the first stanza.I find the opening vamping on Db-Gb-Db with the strings and harp to be a bit slight and almost overly sweet compared to the tune itself--to me, it evokes unfortunate implications of sacro-pop. Your tune deserves a more sturdy introduction, I think.
The descant is indeed on the last (third) stanza. As for a unison tutti with the double descants, I wrote (above):It might just be your style to not put the descant on the last verse--I think, though, that even with your doublings, it's best to set the two descants either on separate verses or together on top of a unison tutti so that both of them and the melody don't get totally lost.
I note also that, with only three stanzas and, given the non-homophonic treatment of the second, there is really only the final stanza available for the S & T descants (which were written to complement each other as well as the melody).Although the choir in the final stanza is scored in four (SATB) parts, it may be advisable or even preferable for the choir (especially a smallish one) simply to sing the melody only, so as not to have the melody of the tune obscured by the S & T descants.
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