But the intent must be to baptise in some understanding of the Christian God.

The Christian God is not the same as the christian god. 
The Christian God = the Catholic God = the Triune God who was born of the Virgin Mary
Most protestants and non-Catholics do not believe in the true God.

But isn't the 'Chrism Mass' itself a Bugnini innovation? In the Missale Romanum published by Pope Pius V after the Sacred Council of Trent, and thereafter until Bugnini, Holy Oils are consecrated during the sole Mass on Maundy Thursday, not at a separate Mass.

Do you have a reference to support that assertion?

Surely 'pre-Bugnini Chrism Masses' per se did not exist. Prior to 1956 in Cathedral churches Pontifical Mass with the Consecration of the Oils were celebrated on Maundy Thursday.

I suppose if the last supper mass is in the morning, there wouldn't be much time for another mass.
The Gelasian sacramentary has separate Chrism and Last Supper masses for holy Thursday.  They must have merged before Trent. 

Taken in toto, the following six links suffice, I believe, to demonstrate persuasively* that before 1956 there was no such thing as a Chrism Mass and that the three kinds of reserved holy oil were consecrated EITHER during the Solemn High Mass on Holy Thursday celebrated by the diocesan ordinary OR in a special ceremony outside of mass on the same day, the performance of which was reserved to a diocesan ordinary or his equivalent (e.g., an abbot).

Note that the stress placed on the preceding either-or construction is meant to indicate, not a permitted alternative, but an apparent contradiction between what is asserted in the second and third Catholic Encyclopedia articles linked below. My suspicion—as I have no probative or even persuasive evidence, that is all it is—is that the third article has been edited or revised at some time during the past thirty years. If, as I suspect, the second alternative is correct, it is probably also safe to assume that the CE's assertion that the special ceremony was instituted in the fifth century is correct.

https://www.britannica.com/topic/Maundy-Thursday#ref1254399
http://modernmedievalism.blogspot.com/2018/03/hail-holy-chrism.html
http://www.usccb.org/prayer-and-worship/sacraments-and-sacramentals/sacramentals-blessings/blessing-of-oils-and-consecration-of-chrism.cfm
https://www.newadvent.org/cathen/07421b.htm
https://www.newadvent.org/cathen/10068a.htm
https://www.newadvent.org/cathen/03696b.htm
_________________________

*Please note the choice of words. I do not claim that the evidence presented is probative, simply strongly persuasive. Insofar as the pre-1956 ceremony is concerned, the evidence presented also corresponds with my own recollections of what I was taught by nuns and priests at that time, when the changes to the Holy Week liturgy were a very big deal for religious and laymen, old and young alike. (In the year 1956, I completed the sixth grade and started the seventh.)

Regarding Baptism, Fr. Paul Trinchard (May he RIP) told us at a conference in New Orleans that many dioceses in the U. S. were using manuals with the intention that the child is born into the world which is in the state of original sin, not that the child had original sin. The society reconditionally baptized 3 of my children. I wasn't taking any chances, as you can't get into heaven without it.