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
cf. Quæstiones antequam q. 15, d. 2, m. 4, n. 47: "Item, proprium summae bonitatis est se declarare per bonitatem creatam; ergo summe declarat se per summum in creatura sicut est possibile. Sed summa bonitas non esset [in] creatura quam contingit intelligi, nisi esset incarnatio: non enim pervenit pura creatura ad illam bonitatem, ad quam pervenire potest creatura unita deitati; ergo conveniebat incarnationem fieri etiam si non esset passio."
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.
(Ps.-)Augustine wrote something similar in De anima et spiritu (quoted in SH 1V, P1, In1, Tr1, Q2, Ti2 (n. 23), Respondeo, p. 42; Hunter's transl. p. 148):
For this reason God became man, that the whole human being might be beatified in him, that humanity might advance both inwardly through intellect, and excel outwardly through sense, that one might find pasture in their Creator, interior pasture in the cognition of deity, outward pasture in the flesh of the Savior.
Propterea Deus factus est homo, ut totum hominem in se beatificaret, ut sive homo ingrederetur intus per intellectum, sive egrederetur extra per sensum, in Creatore suo pascua inveniret, pascua intus in cognitione deitatis, pascua foris in carne Salvatoris.
The Salmanticenses discuss and refute the
argument: Christ the Lord was de facto predestined and came into the world not only for the end of our redemption but also for other ends.
All the other causes [or motives of the Incarnation] […] have to do with a remedy for sin. For if man had not sinned, he would have been endowed with the light of Divine wisdom, and would have been perfected by God with the righteousness of justice in order to know and carry out everything needful. But because man, on deserting God, had stooped to corporeal things, it was necessary that God should take flesh, and by corporeal things should afford him the remedy of salvation. Hence, on Jn. 1:14, "And the Word was made flesh," St. Augustine says (Tract. ii): "Flesh had blinded thee, flesh heals thee; for Christ came and overthrew the vices of the flesh."
omnes aliæ causæ quae sunt assignatae, pertinent ad remedium peccati. Si enim homo non peccasset, perfusus fuisset lumine divinæ sapientiæ, et iustitiæ rectitudine perfectus a Deo, ad omnia necessaria cognoscenda. Sed quia homo, deserto Deo, ad corporalia collapsus erat, conveniens fuit ut Deus, carne assumpta, etiam per corporalia ei salutis remedium exhiberet. Unde dicit Augustinus, super illud Ioan. I cap., verbum caro factum est, caro te obcaecaverat, caro te sanat, quoniam sic venit Christus ut de carne vitia carnis exstingueret.