Am I also tripping on acid in this scenario? That is the only way someone would decide to randomly cover up a toddler's death when they had nothing to do with it. Issac doesn't present as the smartest person but I just don't see him entrapping himself in the hypothetical scenario you paint. If something like you propose happened (it didn't) I would see someone not wanting to get in trouble just turning around and leaving, see no evil hear no evil.
Since Klein and LE have expressed doubts Deorr was ever on the mountain alive, my mind has turned towards those scenarios. I now think the 8 different versions of who was fishing, exploring, hiking, etc are all just a sideshow. I am more inclined to believe the parents hid the body before going camping and fooled the other campers into believing Deorr was there when he in fact wasn't. Perhaps GGP was in on the original plan or maybe he figured it out after not sure, but I see Issac as just being in the wrong place at the wrong time.