I parlanti bulgari riescono a comprendere i dettagli (diversi dai prestiti linguistici) presenti in testi più difficili (non basilari) scritti in altre lingue slave che utilizzano anch’esse il cirillico?

As with any language, if you are not familiar with the words (as you said excluding leanwords) you won’t be able to understand it. I would tell you what those texts are generally about, but that is because the words пушка, снаряд, etc. are in Bulgarian as well. Besides them – no idea.