I’m a bit puzzled by the lady’s choice of the verb dihtan ‘compose, write’ even with the dative þæm to make it ‘direct, command’.
A close translation of “music shall untune the sky”, keeping her other choice of vocabulary, would be hwistlung wille þone rodor unġeswégan since scallan in Old English was ‘to be obliged to’, still a pukka verb in its own right that only rarely and lately began to turn into the auxiliary verb of today for which, I feel, its meaning is a bit too strong.
Yet I know nothing of Dryden either so that takes no account of the style and tenor he may well have intended, too. If the declaration of a wish is intended, one could just put the verb first in the subjunctive: unġeswége þone rodor hwistlung ( “may music untune the sky”). Yet if a tone of command is intended, there's a bitch because the imperative conjugates identically: unġeswége þone rodor hwistlung ( “untune the sky, music!”).
Also it looks to me that midswégan says ‘harmonize’ more clearly than ġeswégan
Is that any use to her?