Conan straight-up thinks Rumi called him over to encounter the murder mystery and he saw less of the situation than we, the readers did via 3rd-person omniscience. Rumi was all dressed and ready to go out and had no reason to be late to school, heard the convo between the soon-to-be killer and vic, then suddenly changed plans, went to a store to buy extra things in prep for the kids to come over, and was late to school. After the tiger shenanigans, she very, very conveniently invited the kids over at the time the murder plot was going to take place, which she overheard during the convo (e.g. paella dinner time). You can complain about figurative post hoc smoke not technically being correlated to fire all you want, but I'm just going keep pointing at the metaphorical firetrucks parked out front of this situation. Let me reiterate, Conan thinks Rumi prepared a setup so they would encounter the murder case. (File 987 pgs 5-6) Do you need Rumi to literally monologue her plans before you believe that she took advantage of an incoming murder situation to get close to Conan/see Conan in action?
So this is definitely truth and there isn't the slightest possibility that this is a convenient lie for her plan?
Your argument is essentially Rumi did one thing I thought was immature, so she might be a teenager in disguise.
I can't take this seriously because it's ridiculous and shallow. I'm getting flashbacks to when someone seriously argued with me that Okiya can't be Akai because Okiya smiles too much.
Well, I'm going to be blunt. Rumi being Sera is a pretty bizarre and unlikely theory. I don't know why you'd invest so much energy in it. I think it's more likely that Wakita taught Amuro how to make ham sandwiches, but I'm not having a 10+ comment reply chain to chase that one.