@Ammako Update: I'm 10 hours in and Rotom asked if I want the nickname Roto-M after defeating the fire trial. I ignored the question for 5 minutes but he was very persistent with it, and always asked at the end of battles and interactions, etc... I tried to get rid of the question by going to the route 8 Colress cut-scene (Rotom usually has some dialogue at the end of cut-scenes), but he still asked the question. So I finally denied the question and went into battle to test whether he gave unwanted advice afterwards. After 2 battles, he did.
I reset the game (and lost about 20 minutes...), and tried to reach the point where I was before to get a save file closer to the question. However, when I touched Rotom the instant after resetting, he asked the nickname question again. I made a copy of the save file then started the game again. When he asked the question I still denied it. I went into grass and had at least 20 battles but Rotom never gave advice after them, even though I answered the nickname question. So I reset the game again, but this time I accepted the nickname question. Shockingly, after many battles, he never gave advice. I saved then reset the game again to see if that had an effect, but it did not--he still never gave advice.
Unfortunately, right before I began the fire trial again, Rotom gave me random advice, as well as right after. I tried battling in grass, but he gave more advice. I saved the game and made another copy. I've put both saves in a zip file--you can have a look if you want to
saves.zip