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[Emphasis added above.]

A cautionary side comment: Be very careful in the use of analogy. This works only as an illustrative didactic tool where there is a clear correspondence, not where certain attributes of the items being compared might elicit an inapplicable premise.

Penance is Penance. Baptism is Baptism. Penance requires reason and the capacity to accuse oneself in detail after purposeful reflection. This requirement does not apply to Baptism per se, as is obvious in the section from Pope Siricius (emphasis added):

So while I'm not prepared to comment on necessity, I will point again to the problem of desire/intent/votum. It's one thing to discuss the generalities of sacraments, but it's another entirely to impute particular votum and then ride upon that. Votum for Penance is its own thing, and it doesn't serve to explicate desire for Baptism.

"This sacrament of penance, moreover, is necessary for the salvation of those who have fallen after baptism, as baptism itself is for those as yet not regenerated."

In His infinite mercy God has willed that the effects, necessary for one to be saved, of those helps to salvation which are directed toward man's final end, not by intrinsic necessity, but only by divine institution, can also be obtained in certain circuмstances when those helps are used only in desire and longing. This we see clearly stated in the Sacred Council of Trent, both in reference to the sacrament of regeneration and in reference to the sacrament of penance (<Denzinger>, nn. 797, 807).

https://www.ewtn.com/catholicism/library/letter-to-the-archbishop-of-boston-2076

It was a cautionary side comment, and it was as to method not as to substance (other than peripherally with regard to distinctions in procedural votum between those two sacraments). As you note above (emphasis added) the parallel is necessity (leaving aside, for the sake of completing this reply, whether that necessity speaks to justification or to salvation).

The ends sought by/for the soul via the two sacraments are the same, yet the means by which a given soul, as catechumen-subsequently-baptized penitent, obtains these ends at two different moments in time are not (re your citation of Pope Siricius regarding adult Baptism). There appears to be little if any question as to what constitutes the sacrament of Penance. There appears to remain some question as to what exactly (once we depart from the usual form and matter) constitutes the sacrament of Baptism.

If A is a discrete set while B is not, then it might be prudent to refrain from stating that A and B in all of their respective attributes (beyond general necessity) are analogous. Otherwise, that would be an opening to avoidable confusion.

As to reliance on Trent, in Reply #13 you said as well:
Might someone not already fully adept in that element of the Catechism infer that the analogy in use here is your reading?

Earlier I also said clearly that I don't pretend to deal with the main topic of sacramental necessity. I leave that to those with the requisite knowledge, whose diligence toward the subject and whose care for the clarity of its treatment I do appreciate.

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I see an argument against my position by saying that the Catechism implies that a BoD would apply to all catechumen in such circuмstances. But I don't think it can be read that way, and think my analogy to the sacrament of penance applies.

Might someone not already fully adept in that element of the Catechism infer that the analogy in use here is your reading?

On adults, however, the Church has not been accustomed to confer the Sacrament of Baptism at once, but has ordained that it be deferred for a certain time. The delay is not attended with the same danger as in the case of infants, which we have already mentioned; should any unforeseen accident make it impossible for adults to be washed in the salutary waters, their intention and determination to receive Baptism and their repentance for past sins, will avail them to grace and righteousness.

The Catechism of the Council of Trent also confirms that the Council taught that for those who fall into sin after baptism, the sacrament of penance is as necessary to salvation as baptism is for those who haven't been baptized.

That St. Bonaventure text you linked to is interesting.  There's also another post on that blog that's relevant to this topic:

https://thecenturion1.wordpress.com/2023/09/08/a-necessary-dilemma-rejecting-baptism-of-desire-requires-rejecting-perfect-contrition/