How would miaphysites approach the question of monothelitism versus dyothelitism?

Miaphysitism is very similar to the Hypostatic Union.

Miaphysitism (sometimes called henophysitism) is the Christology of the Oriental Orthodox Churches. Miaphysitism holds that in the one person of Jesus Christ, Divinity and Humanity are united in one "nature" ("physis"), the two being united without separation, without confusion, and without alteration. (Source)

The term miaphysic is best understood as ''one united nature'' i.e. two natures united in Jesus Christ and thus, it is different from monophysitism (''one nature'' not two natures in Jesus Christ).

Contrariwise, the Chalcedonians saw the Oriental Orthodox as tending towards Eutychian Monophysitism. However, the Oriental Orthodox persistently specified that they have never believed in the doctrines of Eutyches, that they have always affirmed that Christ's humanity is consubstantial with our own, and they thus prefer the term Miaphysite to be referred to as, a reference to Cyrillian Christology, which used the phrase "μία φύσις τοῦ θεοῦ λόγου σεσαρκωμένη", "mía phýsis toû theoû lógou sesarkōménē". The term miaphysic means one united nature as opposed to one singular nature (monophysites). Thus the Miaphysite position maintains that although the nature of Christ is from two, it may only be referred to as one in its incarnate state because the natures always act in unity.

In recent times,leaders from the Eastern Orthodox and Oriental Orthodox churches have signed joint statements in an attempt to work towards reunification. Likewise the leaders of the Assyrian Church of the East, which venerates Nestorius and Theodore, have in recent times signed a joint agreement with leaders of the Roman Catholic Church acknowledging that their historical differences were over terminology rather than the actual intended meaning.(Source)

In my research, myaphisitism is indeed compatible with the dyothelitism of Chalcedon.

The identity of the original human being was known by name. God named the first human beings , the male and female, with a singular name 'Adam'. Scholars say that the name 'Adam' applied to Eve in Genesis 5:2 was a designation of her equality with the male in human nature as stated in Genesis 2:23 where the male said that the female was the flesh of his flesh and the bones of his bones. In this case, God was treating human beings, male and female, as one.

The union of the divine nature and human nature in a singularly named person called 'Jesus' was similar to the spirit being united with flesh which was called singularly as soul (cf. Genesis 2:7).

Consciousness existed in the mind. In Jesus, the human mind and the divine mind were one i.e. in unity/united. This meant that in Him was united minds or one consciousness. This one consciousness had two distinct divisions in which similar but not identical to the Perichoresis of the Trinity, the divine part had full access to the human part whilst the human part had varying access to the divine part -- from limited access during his ministry before the crucifixion to full access post-Easter.

If the divine mind and human mind were two consciousness in Jesus, it would mean two separate persons (Nestorianism) which would be akin to Multiple Personality Disorder (MPD) wherein there existed ''split identities'' (www.webmd.com) in which there was multiple names for each person existing in one body. This was not the case in Jesus Christ. (Jesus Christ: one person with two minds based on biblical Anthropology and the Perichoresis of the Trinity, R. Brown, 2020)

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.