Been on hol, so have only skimmed this thread a bit. But I think I can help out with the 'how do they know that x months is right for solids?' bit.
For a long time, the advice to mothers has been the same, whether their babies are breastfed, formula fed or a combination. 'Start at 4-6 months'.....interpreted, wrongly, by many HVs, as 16 weeks without any leeway.
It may be that formula fed babies do need something else at 4 months - the taste and consituents of formula never change (unlike breastmilk, which prepares babies for family meals by taking on different tastes, and which meets the needs of the growing baby by becoming tailor-made eg by becoming at times, more calorie-dense, and by matching the appetite needs of the baby).
It certainly appears that formula fed babies tend to 'ask' for something other than more milk earlier than breastfed babies, and solids are on the whole given to formula feds sooner than breastfeds.
But when large numbers of babies have been studied for growth and health, it is pretty clear that the majority of babies are fine on breastmilk alone for six months - www.pediatrics.org/cgi/content/abstract/110/2/343 is only one of a large number of papers showing just this.
There are lots of 'clues' why six months is about the right age, from a biological, anthropological and social point of view. Babies can chew a bit then (with or without teeth), and they have some hand-eye co-ordination to hold on to foods and eat them (they don't really need spoons or plates); they are sociable and enjoy joining in activities, such as meals; they can sit, with help, on a lap without much support; they're less likely to choke; they're interested in new experiences like tastes and textures and have started to explore things with their mouths.
There is also some research about allergies and early solids, which means giving solids much before three months or so could be bad news for susceptible babies. I don't think the evidence is that strong for after this time, though there are some papers which suggest six months does give protection.
The stuff about oral development is rubbish - a total misunderstanding of a small study (ref not to hand) of babies who were bottle fed for a long time, and who already had motor delay and other problems linked with a disability. To apply it to healthy breastfed babies (or bottle fed babies) is quite wrong.
Prolonged exclusive bf - say, after nine months or so - does show up as a risk for anaemia but by then the majority of babies are having some solids - they are putting everything into their mouths, and will end up with stuff from your plate anyway. There is no point in deliberately delaying solids after six months....but going at the baby's pace means most babies will inevitably have little tastes and bites from this age anyway.
There is very little academic work done on the nutritional needs of formula fed babies - no long term studies AFAIK to hint at whether four months or six months is 'best'. Common sense would indicate that following the baby's lead is fine, and making sure a variety of tastes and textures are given to build up a repertoire of healthy, enjoyable foods. There is no rush and it is not a race. It is not 'better' to have three meals a day at five months - babies still need milk (formula, breast) at this age. The younger the baby, the more the need for the milk, so too many solids can displace the milk too early (babies only having small tummies!).
Vitamins - vitmain D supplements are already in formula. UK babies probably do benefit from them, and need them in addition to breastmilk from about 6 months...not because breastmilk is deficient, but because our weather and our lifestyles are!! I think I have posted on this here before. In generations gone by, we probably all were outside more, and got our Vit D from light. These days, we are indoors or in the car, and babies in cold weather are wrapped up, so their skin doesn't get the light it needs to make Vit D. This is especially important for black or brown babies who, ethnically, may need more Vit D than our lousy weather allows them.
But you can get Vit D from other foods, and if you and your baby have a walk outside several times a week, with the baby not totally cocooned, you're probably ok.