Apologies for the wall of text and for any possible mistakes...
The classical rule for enclitic (unaccented word following the accented one) position states that the enclitic is ideally placed right after the first accented word (not getting into the details of multiple enclitics and their mutual relationships in placement, but basically they all cluster together). This is historically the most archaic tendency, which can be seen e.g. in the 16th century
bugaršćica poem from Hvar:
Dvȃ mi sta siromãha dugo vrime drugovala... (I underlined the two enclitics, which are both archaic or stylistically marked from the modern perspective, so I won't get into describing their exact meaning; the two accents are my ad hoc "reconstructions", I won't guarantee their 100% correctness). As you can see, the subject phrase
dva siromaha, made up of two accented words, is outright broken up by the two enclitics. (It should be noted that this breakup is not forced by the metre of the poem, because the
bugaršćica verse is fairly loose; I could pull out similar examples from prose texts from the same era, but I like this one and remember it off the top of my head.)
This rule with such multi-word subjects is still somewhat observed in especially fancy writing (more so in Croatian, which had a purist archaicising phase in the 90s and 00s, than in the other BCMS standards, as far as I can tell). So you can even write
Stephen je Spaulding bio vrlo sretan čovjek, and it would not be incorrect (though it might sound masturbatory). But the enclitic in modern usage is overall quite free within the sentence. The classical rule applies 100% to single-word subjects (
Stephen je bio vrlo sretan čovjek, and NOT *
Stephen bio je vrlo sretan čovjek), but when you have multiple words in the subject there are multiple solutions. The second tendency is the one you have come across, positioning the enclitic after the verb (
bio je; i.e. after the first word of the predicate group). And you expected what is indeed the most ordinary variant today,
Stephen Spaulding je bio vrlo sretan čovjek, i.e. the enclitic just follows the subject regardless of its structure. The
je functions as a "logical" step between the subject and the verb. When this word-order was attempted to be introduced to the literary norm, it was
met with some backlash, and Croatian normativists will avoid in writing even today, but it is by far the most colloquial variant.
All these variants are legitimate and fundamentally natural. The choice is a matter of style and desired rhythm. So, to sum up:
A)
Stephen Spaulding je bio vrlo sretan čovjek - the most commonplace, colloquial variant
B)
Stephen Spaulding bio je vrlo sretan čovjek - literary, moderately formal, more likely to be found in writing than in speech
C)
Stephen je Spaulding bio vrlo sretan čovjek - poetic, archaic, in older literature
More complex sentences can have even more variants, because, again, the position is very free. Here's the first sentence (clause) from Knausgård's "Autumn" in Croatian translation (M. Delalić) which caught my eye today:
Iz nekoga (1) razloga (2) voće (3) na sjeveru (4) vrlo (5) jednostavno i pristupačno[...]
All five positions can accomodate the
je, and in the actual text it is found (classically) on (1), breaking up the adverb phrase.
"Stephen Spaulding bio je vrlo sretan čovjek..." sounds so unnatural to me
Maybe from the viewpoint of your colloquial usage, but as far as the "literary" language goes, it's natural.
If you say: Čuveni pesnik XY je bio vrlo srećan čovek. (XY, koji se nedavno oženio, je bio vrlo srećan čovek.) you simply have to stress "je" (there is no other way to pronounce it). Read it aloud and you will see.
Well first of all "read it aloud and you will see" advice is borderline meaningless when addressing a non-native. Their pronunciation is not intuitive like ours. They can stress anything however they want.
Secondly, I'm a native speaker and I would not stress the
je. The
je acts as a proclitic to
bio (maybe in the first example, and definitely in the second example, and that is why normativists will avoid such word order in particular, e.g. see the above link, p. 153:
ljudi koji rade oko napretka svoga naroda su dostojni pohvale), or (in the first example) as an enclitic to the preceding word (
XY, actually the second of the two words, assuming it's name+surname, two accented words). Either way it is impossible to add stress to the enclitic (in the proper sense of the word, i.e. rendering it as
jȅ), unless there's some wider semantic reason to do so. (
Ti nisi, ali, za razliku od tebe, on jȅ pogledao taj film. - which could also be rendered with the ordinary stressed form of
je: on jȅst pogledao taj film.)
