This is to misunderstand the nature of rubrics. Normally, often enough, the rubric or a supplemental instruction will advise the celebrant of the sacrament in question on elements that may be adapted and modified.
As one example from many, the rubric for the anointing of the sick states that the anointing is given on the forehead and the palms of each hand. The instructions say that, in case of necessity, a single anointing on the forehead suffices – and that if the head of the patient is not available, one should get as near to the head as possible.
There have been many times when I am administering the sacraments to one at the point of death where I, as the celebrant, have made the determination that is properly mine to make, that the closest I can get to the patient’s head is the sole of his foot, so as not to interfere with the health care professionals who are dealing with the crisis/trauma that the patient has sustained.
It is ultimately the theology of the sacrament itself that tells the minister that which is essential and cannot be varied. That, along with any other instruction or directive, is the paradigm to be used in interpreting the relative weight of the various rubrics and making prudential applications to the concrete situation in which the sacrament is being celebrated.
As to the original posters questions:
- This would be covered in academic courses as well as during in-seminary practicums.
- In a seminarians pastoral rotations, he should have exposure to this as it occurs in real life
- His formation should cover his staying abreast of the rubrics.
When I was a professor of liturgy and sacraments, how the courses for the individual sacraments were formulated and where they came in the multi-year programme of study varied.
As to why your pastor did this – I can’t answer that since I can’t know what was in his mind. Why don’t you ask him?
Personally, when I do this, it is a bit of a judgement call on my part. If I can access the crown of the head, I do so – all else being equal and the child is not thrashing about.
If the baby (or sometimes we are beyond the stage of an infant) actually has a lot of hair, I will assess whether I am actually more anointing hair as opposed to the head and I may adjust for that so that the chrism is actually in contact with skin as opposed to hair.
If the infant is extremely agitated, that also enters into the equation – since the infant has just had water poured upon it and may not be happy. The anointing with chrism is, by definition, an explanatory rite in the celebration of this sacrament…along with the conferral of the candle and the bestowing of the white garment.