I suspect that stop and fixer are in for replacement due to contamination, though only 5 rolls have been developed in it, while 10 to 15 should be possible. I'm considering to make an extra rince between development and stop bath and between stop- and fixer. anyone have experience with extra rinces between baths?
if i'm not mistaken, 4 minutes in ilford rapid fixer (1+4) at 20°C, 10sec (4-5) inversions every minute.
end-rincing is manually: 4+10+20+20 inversions in 20°C water, with the last a wetting agent added.
The Ilford-esque washing procedure may have something to do with it, but honestly, I'd not worry about it. The negs should print just fine.
I might add a hypo clearing agent to your process, however. I might insert it after a rinse following the fixer, and followed by another rinse before beginning your inversion washing sequence.