UPDATE: The following post is an updated version of another (found here; it's short, you should read it for a proper backdrop), but due to length of the corrective explanation, I decided to just make an entirely new post.
After chatting with my good friend Josh my thinking on this question got clear, real quick. As it turns out, I was quite confused. So now I'd like to share with you a more accurate way to approach this beast...(any inconsistencies of thought or mis-thoughts are mine and shouldn't to be attributed to Josh's assistance to my analysis—but I give him all the credit). Thanks Josh!
Without אשׁרי in the target verse there would be no predication, there would simply be a noun phrase (i.e. a forgiven person). So without a doubt, אשׁרי predicates something and is naturally, then, to be called the predicate.
However, we do ourselves a disservice when we naively slop on other grammatical categories—such as 'interjection', 'vocative' or 'direct object'—that do not exist universally across languages. It's important to remember that not all languages are the same. Each have their own ways of "making sense". Nonetheless, through sweeping typological studies of many languages we have realized that there are shared systems in which languages may be found to operate.
For instance, one language may primarily use word order to indicate grammatical roles—such as subject, predicate, or object (e.g. English)—while others may prefer the option of a morphological case system (e.g. Latin). The former may be called configurational languages while the latter are simply dubbed non-configurational. Conversely, word order variation in languages like Greek (a non-configurational language) is normally used to specify pragmatic relations (i.e. topic or focus) since the grammatical roles are already taken care of via case endings, while configurational languages, like English, signal pragmatic relations via prosody as word order does the trick for spelling out the the grammatical roles in play.
With this said, it should be noted that Biblical Hebrew is a non-configurational language utilizing word order variation to mark various pragmatic relations (i.e. topic and focus).
Notice how we can talk about speech propositions from so many different angles (e.g. grammatical, pragmatic, etc.) without using language-specific terminology, instead, making use of other universal, descriptive categories.
So rather than go down the rabbit-trail of trying to appropriately label אשׁרי according to non-universal categories, it seems far more promising to employ some typological tags, like those mentioned above. For the purpose of looking at אשׁרי, we can talk about it grammatically (viz. it is the predicate of the utterance) and in terms of its pragmatic role as far as information structure is concerned.
We then no longer discuss אשׁרי in terms of whether or not it is a proper 'interjection', but according to what role it plays in these typological slots; and as we've already stated, that it predicates something about someone is clear. Without the meaning encoded by אשׁרי the utterance would mean nothing coherent at all. So already, we've ridded ourselves of the conundrum of dealing with a so-called "interjection" that contributes some predicative force to the sentence.
Having disregarded this notion that אשׁרי be discussed in terms of English categories, like 'interjections', we may now discuss its word order. As a piece of information that is structured in a particular manner in the sentence, we may then ask why it is so situated (for we can't discount it simply because it's—supposedly—an "interjection"). Yet on this note, it is important to remember that language use has a way of crystalizing key expressions, go-to formulas, and stock phrases. Such idiomatic lines, after repeated use and repeated use, become fossilized in particular formats that aren't to be questioned and only to be changed if wit or need calls for it. In these cases, then, the word order of whatever formula phrase is chosen cannot really be evaluated in the same manner that a more "lively" construction could be esteemed (viz. one that allows its constituents to be moved around). "Encrustment" must be accounted for—entrenchment appreciated.
This means that as far as 'information structure' is concerned, constituents that do not allow the speaker/writer an option for where to place them or how to use them likely forfeit any impact, as far as 'markedness' is concerned, they might yield through word order variation. After all, choice implies meaning—so where there is no choice, we must be cautious in our evaluation, not assuming the typical effects/functions of information structure are communicated with fixed expressions (this doesn't mean there is no meaning or pragmatic function present, just that it might not be what is regularly conveyed through such an arrangement).
Now let's wrap this up: for אשׁרי this means that because it always stands at the front (45 of its 45 occurrences in the Hebrew Bible) of a clause, the fact that the predicate is at the beginning of the utterance in Ps 32.1-2 does not mean it carries any pragmatic effect, such as predicate focus (for this would be typical of a nominal clause structured Predicate-Subject since the default/unmarked order is Subject-Predicate). In other words, the predicate indicated by אשׁרי is most likely not indicative of marked word order—it simply is what it is because that's the way it is. This is not to say that אשׁרי is not the 'focus' of the utterance (in the technical sense of information structure)—for the predicate is the natural recipient of focus since it is usually the harbinger of new information—but simply that it is not a case of marked focus.
There are cases of course that break the mold—at least superficially—in which a constituent is placed before אשׁרי, but only three (Prov 14.21; 16.20; 29.18); and in all three instances, the fronted constituents are cases of nominal left-disclocation and thus stand outside the clause, so really, אשׁרי is still standing at the head of the clause (viz. left-dislocated constituents are not clause-essential elements).
Having said all this, I think we are in a way better position to answer my original question—which now needs some technical tweaking!
Is the arrangement of "is truly happy" of the CEB translation reflective of the information structure as found in the Hebrew text? [But there's another question we must answer now, concerning the pragmatic relations (i.e. topic and focus) of the CEB's translation…]