Could one also substitute “begab” for “ergab”, and thus give this sentence an archaic / poetic flavor but with more-or-less similar meaning?
ergab in this sentence means sth like ‘turned out’ so no.
In English, saying “It happened that…” and “It turned out that…” are more or less synonyms. Saying “It turned out…” places more emphasis on the result or outcome of whatever happened, and connotes that the outcome was perhaps contrary to what might have been expected, whereas saying “it happened” places more emphasis on the action / occurrence itself, and lacks as strong a connotation of being “contrary to expectation”. But this difference is subtle and in many cases it would be completely interchangeable. I take it in German these meanings are a bit more different? There’s something I’m not getting here then.
ergab and begab are not interchangeable.
Ergab concerns, as it turned out the result more than the process and it does this so strongly that substituting it with begab changes the meaning altogether. If I use ergab, even though its alway in a passive construction, I imply active personal involvement, be it in action or just in understanding. Nothing ergibt sich without anyone doing anything. Begeben on the other hand is strictly objective. It does not imply any involvement from my side.
In this context, I understand the sentence to be said by a police officer etc. who reports on his findings on a suspect. If ergab is replaced with begab it not only becomes a bit archaic but also changes the meaning completely, from a personal, involved narrator to one that is telling something he most likely wasn’t involved in.