interesting.
i wonder, then, if poor fuel quality caused--somehow--the ECU to induce a lean condition. i believe a lean condition raises combustion temperatures which, in turn, raises exhaust temperatures in excess of the operational efficiency temp range of the cat (900°F-1400°F) which, in turn, can trigger a p0420 DTC.
but still... bad gas..? is that really a thing? nevertheless you want to be genuinely prudent in this regard, so you might consider visiting pure-gas.org and locate a source of ethanol-free fuel for your Accord. it'll be a pricey but an effective method to both purge and prime your fuel system. the j-series engines are indifferent performance-wise to the octane value of fuel; however, they do perform very nicely on the non-ethanol stuff.
i'm suspicious bad fuel is really the cause of your p0420, however.