Are there any theologians (Doctors or Fathers of the Church) who thought that, regardless whether Adam had sinned, Christ still would have incarnated, primarily to reveal the doctrine of the Trinity?
Hugon, O.P., summarizes
Alexandr. Halens,, Sum. Theol. [Summa fratris Alexandri], dist. 3, q. iii, mem. 13
in Tractatus Dogmatici (vol. 2) pp. 310-11, de Incarnationis motivo:
Alexander Hales proves the Incarnation, even excluding the motive of the redemption, to be still most fitting, because God the supreme good maximally diffuses himself* and blesses the entire man—namely, both his intellectual and sensory parts.
Alexander Halensis id solum evincit Incarnationem, etiam secluso motivo redemptionis, esse adhuc convenientissimam ut Deus summum bonum se maxime diffundat et totam humanam, partem nempe et intellectivam et sensitivam, beatificet.
Thus, Hales believed that the Incarnation contributed to maximally blessing man's intellect.
It would be interesting to see if Hales cites John 18:37 ("for this came I into the world; that I should give testimony to the truth") or John 17:3, which alludes to the Trinity ("Now this is eternal life: That they may know thee, the only true God, and Jesus Christ, whom thou hast sent."), since the Trinity is a truth necessary to believe for salvation.