I see no harm in putting instrumentals in the middle of ballads, same as any other song really, though not necessarily the Irish Jig approach taken by Steeleye Span. This raises another issue about ballad accompaniment, interpretation & arrangement which might be worthy of discussion.

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As for Professional Troubadours composing ballads - well, in one fell swoop such an idea wipes out hundreds of years of exacting craftspersonship in the ballad-making genre. Whilst I argue for master song-makers, I don't see them as being in anyway aristocratic professionals (as the actual Troubadours were), rather everyday working class men & women with a canny knack for shaping a song within the genre of what we have come to think of as The Tradition - which was, just as any such tradition, essentially a creative one.   This ongoing singing, song-making & sharing & modifying & re-invention accounts for the mechanics of the Folk Process, just a shame their names got lost along the way, as is the way with folklore & oral-tradition even in our own day & age.