I think the rule is that the main ingredient has to be used, and if it can't be obtained, the next mentioned substitute has to be used. And if that substitute can't be obtained, then the next mentioned substitute has to be used, and so on.
If this rule isn't followed, then there will be reduction to the bare minimums as you said.
In fact, this rule even applies to alternate forms of yajnas or karmas that are shorter and simpler than the main one.
According to the Manusmriti:
If a man performs an easier yajna that is enjoined only for times of distress when he is not in distress, then he does not obtain any fruit from it.
Moreover, this same logic applies to prayaschittas. The shastras give various prayaschittas of varying grades of severity for a particular sin.
Manu 11.72 - The Brāhmaṇa-killer shall, for his atonement, build a hut in the forest, live there for twelve years, live off alms, and making for himself a flag consisting of the head of the dead man.
11.73 - Or, he may offer the Aśvamedha, or the Svajit—the Gosava, or the Abhijit—Viśvajit, or the triple Agniṣṭut.
11.82 - Or, having confessed his guilt before the congregation of the gods of Earth [brahmanas] and the gods of men [kshatriyas], if he bathes at the Final Bath of the Horse-sacrifice,—he becomes absolved.
The very first prayaschitta listed for brahmahatya is the 12 year penance in the forest. The substitutes for that penance are listed in the subsequent verses. As you can see, they get easier. Now the question is, are all these options equally valid? Can a brahmana killer just wait for an ashwamedha yajna to finish, and go bathe with the priests at the end of it to absolve him of his sin?
According to Mimamsa rules of interpretation, the answer is no. The resolution is, the easier substitutes are meant for unintentional or accidental brahmahatyas whereas the more severe penances are for intentional or premeditated forms of murder, because the logic is that if all these options are equal, who would pick the harder penances? Since no one would pick the harder penances, this would render these expiatory injunctions useless. If they are useless, then why are they enjoined alongside the other penances in the same text? So the resolution is that they are for different degrees of the sin:
[Manusmriti verse 11.73] is quoted in Parāśaramādhava (Prāyaścitta, p. 405), which adds that the various alternatives here laid down are to be understood to vary with such circumstances of each case as that of the act being intentional or otherwise, the person killed being learned or ignorant and so forth