I see, so Hanyuu physically materialized herself in front of Keiichi and his dad, but they both forget about it when she disappeared. The only issue I can find with this theory is that in the Tsumihoroboshi-hen tip "The Reason we moved to Hinamizawa”, it’s from Keiichi’s dad’s perspective and he is reminiscing about his trip to Hinamizawa which he says was from several years ago. During this memory of his he remembers finding two girls playing in a field and he even says “I still remember how they looked”, and obviously those girls he’s referring to are Rika and Hanyuu. So it seems like Keiichi’s dad’s memory of meeting Hanyuu was not erased. Although, I can understand interpreting Hanyuu saying "I haven't shown myself in front of people since that time.”, as not entirely literal like you explained.