By way of background, Miaphysitism dealt with Christ's nature. The Monothelitism controversy was about Christ's will. Miaphysites wanted to preserve the unity of Christ's person by affirming his divine and human characteristics were so perfectly combined as to constitute "one nature." Thus they had trouble accepting the Chalcedonian formula that affirmed "two natures" in complete harmony.
"Would miaphysites consider dyothelitism an expression of Nestorian heresy in dyophysitism, when confronted with the question in theory?" Probably so, because this controversy followed a rather straight line from the Monophysite question through the battle over the Henotikon that sought to settle the "two natures vs. one nature" controversy, though the Monothelitism controversy.
My answer to a related question may shed further light.
The OP also asks: "since in Christ the divine and the man exist unseparably without blending, diluting, or diminishing... there should be one will that is an expression of this perfect union. Is this correct according to miaphysites?" As far as I know that was the position Monothelitism, and miaphysites would tend to agree with it. However the Pope had allies in the East, and some miaphysites would decide to follow teaching of the Roman Church, which rejected Monothelitism on the grounds that in constituted a thinly veiled attempt to undo what had been decided at Chalcedon. The Sixth Ecumenical Council thus affirmed that:
accepting the teaching of... blessed Agatho, Pope of Old Rome, we believe that in our one Lord Jesus Christ, our true God, there are two natures unconfusedly, unchangeably, undividedly, and two natural wills and two natural operations [energies]; and all who have taught, and who now say, that there is but one will and one operation in the two natures of our one Lord Jesus Christ our true God, we anathematize.
So to get to the answer to the main question: miaphysites would tend to approach the question of Monothelitism versus Dyothelitism similarly to the way they and the monophysites dealt with Christ's "twoness" generally. The monophysites affirmed the oneness of Christ's nature as divine. The miaphysites affirmed the Christ's full divinity and full humanity as one nature, not two. The monothelites affirmed the oneness of Christ's will with that of the Father in spite if his two natures. However, even with occasionally strong imperial support, the various "oneness" parties had to give ground to the findings of ecuminical councils and the teaching of the popes.