Even after re-reading I'm confused. I was happy to chalk up Tamayama's appearance to be just Sata's delusion, the problem is, as far as we know, Sata only met with them at the end, unless the end wasn't the end and was before Sata raped Kiwako.
This would explain Tamayam's appearance (it's also important to note, that when Sata met Tamayama it was in an abandoned building, but somehow he left despite being the middle of the day, additionally just after Sata left it was shown that the building wasn't abandoned and had people inside, so it was clearly his delusion). This also explains why kiwako isn't burnt like she was shown to be.
The problem I have with this, is that in his way there we see how both the police and the yakuza get finished off, done by either Tamayama or Mori having gone crazy, unless that was supposed to be the real ending, while what followed happened before, but this would mean we don't actually know what happened to Sata in the end.
And another problem is that even if I can buy that Tamayama is just on Sata's mind, it doesn't explain Kiwako's actioins with seeing him (assuming that can be trusted) and how she took the metal (which I think can be trusted).
I think that the ants somewhat symbolize escapism, wee see them on Mori as he seems to feel remorse after killing Komatsu (his stomach cramps started after his murder as well), so when Sata is tied to the bed being raped and he sees the door open and a swamp of ants coming might him running away from the fact he killed the dancer. Additionally, although it wasn't said his father committed suicide when his mom ran away, ad some point relatively close to the end we get to see a panel covered in ants with kid Sata crying, and an adult hanging which I presume is his dad after Sata's mom ran away.
One thing that isn't clear is how Sata and Kiwako were found together after the rape. The way Sata remembered the encounter was that the encounter was at least semi-consensual, then Kiwako lifts the ax (which to be fair he shouldn't be able to know) and lightning strikes, which would lodge the metal in his skull, but doesn't explain how they both were found separately. The only thing I can think of is that after rape-choking Kiwako, Sata took the metal and left, because several lightnings happened that day it's possible they both were hit by separate lightnings.