Deck, I'm going to C&P to answer your points.
"My reading and discussions led me to believe that when a baby/child reduces feeds, the active immune agents (brain... antibodies?) don't reduce in quantity per 24h but tend to concentrate into the fewer feeds."
I think you have misunderstood. There is some concentration of antibodies when there is less breastfeeding, but this happens with very well-established breastfeeding, and nature assumes the less breastfeeding is happening with an older baby/toddler/child whose immune system is more developed/robust than a small baby's. It's absolutely not the case that switching to (say) half-breast, half-formula, allows the baby to have the same quality and quantity and range of immunity as he would if he were fully breastfed. Like I say, what have you been reading to indicate anything else?
"Also the dynamic infection exchange from baby's mouth to mother's areola continues, to stimulate mother to make the right antibodies in the first place."
Indeed - but this does not work as extensively as it might, if the baby is not fully bf. But the 'exchange' does continue, you are right.
"Now, I think mixed feeding should be promoted because I think it would raise bf rates:"
That is highly speculative. The evidence we do have - a lot of surveys, including our own internationally-respected UK Infant Feeding survey - indicates the absolute opposite. The sooner infant formula is introduced into a breastfeeding relationship, the less likely there is to be any breastfeeding within a short time (the UK Infant Feeding survey consistently shows this - a bf baby who has any formula introduced in the first week is much less likely to be bf at all a few weeks later. It's not pure cause and effect, of course, but it is not encouraging.)
Now, there has been one study which gave small amounts of infant formula to 'problem feeders' among bf babies very early on (at 2-3 days, I think), deliberately to see if this increased the length of time the mothers bf for. The methodology was truly awful, the sample tiny, and the results inconclusive.
It may be the case that in individual cases, judicious use of infant formula where there are bf problems may be helpful and enable the mother to continue bf - as you rightly say, going back to full bf remains possible anyway. But as a basis for promoting mixed feeding, we have no evidence at all that it would do anything at all except reduce bf.
"At present the message is that everybody should breastfeed exclusively for six months. "
No - this is not the message. The message is that the best health outcomes are seen in babies who are excl bf for six months. There is no instruction to individual mothers that 'everybody' should do this....though of course people take it as such. The 'instruction' should be seen as a call to agencies, HCPs, employers, matenity services, to enable women to do this.
" But it isn't necessarily attainable by everyone, whether for physiological, psychological or social reasons"
Quite true.
So we want individualised support and enabling for everyone, full and frank info for everyone, and HCPs responding to what women say they want to do.
I do not believe that many women are unaware that some bf babies have formula as well. They may not know the term 'mixed feeding' but as 99 per cent of all UK babies have formula milk at some point, I am confident the majority of them know this form of feeding exists.