To get meaningful (rather than infinite) values in that standard output table, remove ELECTNUC from the PHOTONUC card.
Note that the formula will give you the reaction cross section, including all reaction channels, at the average beam energy. Therefore, no momentum spread should be input.
The former post, you refer to, mentions the nuclide yield obtained by RESNUCLEi, and not the specific activity (RADDECAY and associated cards are out of context for this purpose). For the related normalization, of course what matters here is the number of beam photon (and not proton) reactions. This is given towards the end of the standard output file as
Number of stars generated per beam particle by PHOTON
The PEATHRES/IONSPLIT PHYSICS cards are obsolete since a while and should disappear from any input file.
Moreover, the desired cross section was already provided in this forum, as you were earlier made aware of.