conker, the guidance on milk only to 6 mths is not 'a fashion'. It's wrong to dismiss it in this way.
We now have a solid, robust, and in-depth scientific literature from all over the world that shows what works best, from a health perspective, in the majority of babies.
In addition, we now know more than we ever did about the physiology and developmental needs of infants. The concerns that babies would need more iron before 6 mths if not already on solids are no longer valid - and this was never a real concern in the developed world, anyway, where the nutrition of mothers was generally pretty good, and certainly normally sufficient to allow a baby to be born with his full 'complement' of iron stores.
Of course all babies don't hit the 'need' for solids at exactly the same date on the calendar. But 6 mths is a useful benchmark, and (more than your argument) is at least evidence-based. There is no evidence that any normal, healthy milk-drinking baby benefits from solids as young as 12-13 weeks, and no evidence that this is linked with sleeping through (in fact, the evidence we do have shows it makes no difference). Any apparent co-incidence is just that - a co-incidence.
In practice, babies will vary quite a lot in how quickly they move on to a range of foods, but round about 6 mths or so is a handy place to start.
If you are reading anything that says babies need 3 meals a day by this time, then your reading is out of date, quite simply. No doubt you could find all sorts of dubious stuff in books and leaflets - even among the ones printed recently.
It is tenuous to say the least to link the developmentally-normal mood changes of a baby (from being grumbly to smiley) to his diet - babies do this, and often they are a little more 'fussy' before a developmental change. Again, a co-incidence.
Most healthy babies will avoid obvious, serious effects of early solids, or get over any milder ones. But you can't argue that a baby of a young age actually needed solids, and to use the perfectly developmentally-normal behaviour of waking in the night as a 'sign' for the need of solids is to mislead people.