I think its mostly because Mordred is still in shock towards the news, so they spend that moment trying to figure out 1) just how much Accolon knows, or condones and 2) is still trying to figure out how they feel about the event. In chapter 5 however, you're able to decide how you feel about him (though the options you get depend on what you choose your relationship with Morgana to be.) Though I do agree- I would like to have more options to express frustration or our feelings to Accolon after the reveal, though that's probably something best mentioned on the tumblr!
I think you're right, it just struck me as a tad bit odd considering you get to react almost immediately to the first reveal and can express it (may it be forgiveness or anger for a while toward both Accolon or Morganna) but have to wait a while (in game) to take any kind of decision for the second
Oh I rarely go to tumblr, I'll go take a look, thanks !