If you want to "provide" the chance for story resolution but you don't want to step on any toes, then do it in a way where you are giving the player the chance to decide, "I can either summon what I want to win a fight, or summon this mysterious being to find out what is going on and possibly still win a fight with it."
So let him begin casting his Gate spell. Tell him something like, "You catch a flickering image of a Marut trying to exit through the gate, but between the flickering images of the Marut, you see flickering images of a magnificent well-armored celestial that seems to also be trying to push itself through the gate (the Trumpet Archon father). The spell seems to be trying to force the resisting Marut through the gate while simultaneously trying to restrict this willing angelic winged creature from pushing itself through. Between the flickering images of the two creatures, you think you hear the winged celestial call out your name. The spell only allows for 1 creature to pass through the gate. You chant the verbal words to your prayer with more focused determination to force the spell to completion. Do you call the Marut through the gate as intended, or do you hold him on his home plane and allow what appears to be a Trumpet Archon to step through as it seems to want to do?"
Something like this will give the player his own choice. Sure, you might be manipulating the player to call for the Archon by making it sound more important, but it will be his own choice. That way he can't complain and if he chooses the Marut, then you know for sure he doesn't really care about the side-story and just wants to whoop butt with a Marut.