So, as John says, seems like I exhausted all possible issues except a bad IRV2. I bought another one as I plan to expand the layout to need 2 eventually anyway. When I swapped out the IRV2, I have exactly the same behavior I had before. The odds of getting two bad IRVs in a row are very slim, so now I'm back to trying to find what's wrong with my setup. I just don't know what else to try!
Does anyone have any ideas? I can't find anything with Lionel like a customer support number (only for returns). Should I call that number or the seller and ask for some guidance?