What if Reimu loses?
Of course, that outcome is absurd. If she lost here, she'd be dead, and there wouldn't be a Touhou Project anymore.
But...supposing she did?
Her soul would be trapped in Hell, slowly transforming into some type of vengeful spirit.
There has long been fan speculation that Kongara is the 4th Oni based on the 'horn' in the middle of her forehead. But, that 'horn' looks a bit like a metal spike- plus there's blood around its base. And we already see from WaHH ch. 27 that Reimu uses very large needles as weapons. What if Kongara is just Reimu's vengeful spirit, killed by her own weapon?
Edit: As to those rice balls, that particular type of rice ball is called 'onigiri', which sounds like the kanji for 'oni' and 'cut'. Those two kanji put together mean 'oni killer'. It's a bit of a taunt, like saying "come at me, bro". Hence Reimu's surprise, followed by annoyance.