rěklo, rijetlo, ricati – I would interpret as source/sourcing, thus the construct po (negation) + ricati, builds poricati = to negate the source, namely deny something.
Pod-rěklo as well as Pod-rijetlo – however builds the construct 'from which source'. In Serbian it is common to lose the unpronounced letters, thus Po(d)rěklo > Porěklo
No. Serbian ortography (and Croatian) is phonological it is truth, but pod is somewhat different than po. It is allreead written in that dictionary:
Promjena prefiksa izazvana je promjenom značenja toga glagola u značenju »nijekati«. Prefiks pod- izražava potajnost (podmititi), kako se i daju nadimci.