As far as whether for every predicative complement (PC) there is a corresponding noun phrase (NP) which denotes exatly the things that have the property expressed by the PC, the answer is yes, this is always possible.
It is of course hard to prove a negative (that something definitely doesn't exist), and for all I know, there could be obscure counterexamples. However, as a rule, there is indeed such a correspondence.
Here is why. PCs are either NPs (in which case we are done: the NP that appears as PC is the required NP), or bare role NPs, or adjectives.
In the case of adjectives, one can always use the same adjective to build an NP (at least I can't think of an example where that's not possible). So the people are afloat leads to the NP the people who are afloat, etc.
And in the case of bare role NPs (phrases that would be NPs if they had a determiner), such as treasurer in Pat is treasurer, as best as I can tell, these are always count nouns (treasurer, secretary, president, and so on), and so one can simply use their plural form to make the required NP (e.g. treasurers).
I don't see how mortal could apply to non-living things. No dictionary I looked at would support such usage. The adjective you actually want is more something like impermanent or transient (see e.g. Lexico).
We saw that to every PC there corresponds an NP denoting the objects that have the property expressed by the PC. There is, however, in general no guarantee that there is single noun denoting the objects that have the property expressed by the PC. It is usually hard to prove a negative, but in technical fields the absence of alternatives is more obvious. So here is one definitive example of an adjective without such corresponding single noun (i.e. without a single noun denoting exactly the things that have the property described by the adjective): continuous. We can say that a function or a mapping is continuous, but English mathematical vocabulary definitely does not have a corresponding noun denoting just the objects that are continuous.
In your particular case, there is an archaic sense of a single noun that has the required meaning: creature. According to Lexico, that noun has, among others, an archaic sense whose meaning is anything living or existing. Lexico gives this sample usage:
dress, jewels, and other transitory creatures
I don't have a handy source for what I'm about to say, but I believe this usage has to do with the notion that apart from God, everything else is created (by God), and so everything else can in principle cease to exist. So we are all creatures in the sense that we are created, and thus finite. According to some (many? most?) theologies, our souls are not transient, and time alone will not make them cease to exist. But God could.