I had some time last evening to read through Mastricht. He also considers the hypostatic union to have taken place at conception in which Christ was filled with all habitual grace:
Here is another line of thought which is necessary to properly understand what was being taught.
Matthew,
I had wrestled with that passage of Mastricht when preparing the article, and I reference it in the article on p. 10, fn. 35 at the end. But I do believe that passage is less clear, and open to different interpretations, than a few other passages of his that are more clear, detailed and longer. Hence, Michael Spangler and I came to the opposite conclusion (after I had initially taken yours; assuming Mastricht was not inconsistent, which I don't see necessary evidence for), that Mastricht did intend to teach that Christ's personal union occured at around 42 days after conception.
Here are some of the other relevant passages (namely vol. 4, bk. 5, ch. 10, sections 5 and 28):
"Moreover, the operation of the Holy Spirit signified in coming upon and overshadowing
had in general the following components, that: (1) he separated some
particle of the Virgin’s substance, from which the body of Christ was formed
(Heb. 10:5). (2) He bestowed a molding force upon the separated particle, by
means of which the Virgin’s seed alone could do in the conception what from the
order of nature both seeds, the male and female, can do. (3) He as it were cleansed
the seed of the Virgin, not indeed from moral impurity or sin, inasmuch as a seed
not yet ensouled is not liable to that, but from physical intemperateness, from
which, in its own time, sin could have resulted, or at the least he preserved the
birth from all impurity, to the end that what would be born of her would be
holy (Luke 1:35). (4) He gradually formed that seed of the Virgin into human
members, in the way that they are formed in ordinary generation (Heb. 10:5).
(5) When the body was already formed, he joined to it a rational soul (Zech.
12:1). (6) In uniting the soul to the body, at one and the same time he inseparably
joined the divine person to both united parts." - pp. 298-99
"but they [the Medieval Scholastics] want the formation of his members to have been completed not only after forty-two days, in
the usual manner, but in a moment, without delay or succession....
The Protestants, with several of the Scholastics, think that it is more agreeable to
the Scriptures for the formation to have occurred successively, because: (1) in the
history of the conception, gestation, and birth of John the Baptist the ordinary
time is noted (Luke 1:38, 56–57), nor is anything different observed concerning
the conception and birth of Christ (Luke 2:6). Therefore, since from these
things they admit that the preparation and development of his body occurred
successively, there is no reason for them to invent something extraordinary in the
formation. (2) In the assumption of the human nature, which occurred through
the conception and birth, it is said that he was made like us in all things except
for sin (Phil. 2:6–7; Heb. 2:14–15, 17; 4:15). (3) The body of Christ when he
was born grew outside the womb of the blessed Virgin according to the manner
of others (Luke 2:40, 52). (4) Miracles ought not to be invented rashly beyond
and outside of the Scriptures.
On the contrary, most of the papists urge for their position: (1) that the Word
assumed a human nature, not an unformed mass. I respond, We judge that the
union with the divine person did not occur before there occurred a delineation
of the organic parts and the union of them with a rational soul. (2) That the
Holy Spirit could have formed him in a moment. I respond, It is not valid to
argue from what can be to what is: the Holy Spirit could also have accomplished
the separation and preparation of matter, and the development after birth, in a
moment. (3) That the first Adam was formed suddenly, and so then the second
also was. I respond: (a) Neither was the body of the first Adam formed in a
moment. (b) In that brief span of time in which the body of the first Adam
was formed, it achieved its full stature, whereas the body of the second Adam
achieved the fullness of this stature successively, as even our adversaries confess.
(4) That if the body of Christ was not formed at one and the same time, the
Word either was united to a body not yet formed or human (which, as everyone
acknowledges, is absurd), or, if the Word was not united to it, this body
existed unformed without the Word. I respond, It existed not yet formed, just as
it existed when it was being prepared, and even before it was being prepared in
the conception, that is, in its causes. But it did not subsist, just as it also did not
subsist after the union with the Word; nor before the formation of the parts, or
before it was a human body, did it exist personally sustained by the Word, as it
began to exist when it was united with the Word, which happened at that time
when the body was at last formed, and made a human body." - pp. 324-25
As to your quote, the Latin, which is not any different from the English, is
here. But how one understands the commas, and what is going on with the understanding of time and the elapsing of it in the verse, and the verbal actions, and what the "first moment" refers to, may be variable.
For instance, it may be that the "first moment" of the broader conception process, when the personal union was accomplished, then Christ was filled with all habitual grace. That is consistent with the rest of what Mastricht says; but holding that it speaks of the absolute first moment of conception would contradict what Mastricht says elsewhere, and I don't even think it would make sense with step one above of the 6 step process that Mastricht outlines.