Here's a link about normal infant fussiness. Could it be just the witching hour in the evening, or do you think there are other times when baby isn't feeding well? Colic and food intolerances a possibility, but I would lean towards normal fussiness first and foremost. DD's didn't settle down until at least 12 weeks, perhaps later, iirc [haze of exhaustion]. Following SWMNBN could pose a problem, not sure it would cause this sort of behaviour, but would really encourage them to drop it and just do everything on demand. This is probably rubbish and I'm sure you know much more than me, but I hope I helped a bit!
Klaw - I would investigate th possibility that milk supply has not kept up with the baby's needs.
Reliance on scheduled feeds can lead to a marked drop in supply after a few weeks. The mother's milk supply can be generous enough at the beginning, and can withstand the impact of scheduling, but after a while, it falls apart....
could not just be supply could also be that is baby that wants to sleep +feed little and often during the day so fussiness could be that baby is overtired at the evening feed having been kept awake too long in the late afternoon.
I start tomorrow and we ditch SHMNBN! Hopefully mum will be content with the natural routine that she and baby find. She was terrified of it all going pearshaped but acknowledges that she is struggling now, is stressed and anxious so accepts that baby led feeding is worth a try
Mum and dad are both lovely, and baby is a total cutie!