It doesn't have to be sitting up completely unaided. It can be with some support because, as you recognise, some babies can't sit completely unaided until 8 months or so. They do need to be able to sit up at least with support though to minimise the risk of choking. In a highchair, with a cushion behind them etc. As long as they're not flopping about or lying back. Of course this is alongside the loss of tongue thrust reflex and ability to accurately pick up and put food in their mouths, chew and swallow.
The developing worlds thing is a red herring. The guidelines are based on maturation of the gut and are provided by the World Health Organisation. Biology is biology. What we know is that the external sounds of readiness seem to mirror the internal readiness of the gut. It makes logical sense that a baby would be able to pick up and put food in their mouth, chew and swallow at a point when they're ready for it. When you talk about giving foods when it feels natural, in my mind there's nothing more natural than allowing a baby to explore food, put it in their mouths, chew and swallow. Most babies are ready to do this somewhere around 6 months. In my mind this IS true baby led weaning. Letting them eat when they're ready. And the guidelines don't say wean all babies at 6 months btw because all babies are ready at different times. They say watch your baby, look for the ACTUAL signs (they also explain the myths around false signs ie alert, chewing hands, watching you eat, waking at night etc, they chew everything because their mouths are where most of the nerve endings are and they chew because they're teething or to self soothe or just to explore the world, they watch you do everything, cook, drive etc, they wake because they're developmentally programmed to, none of these things are signs). The guidelines recommend watching your individual baby, looking out for the signs, going slowly and just happen to point out when their bodies seem to be ready for more than milk is around 26 weeks. Some babies are a bit before this.
I am a little concerned that a hv is going to such great lengths to convince the mothers around her to go against the guidelines.
I took a fairly relaxed approach to weaning. I let my dc make the decisions. They joined us at mealtimes and when they were ready, they reached out and helped themselves. Properly baby led!
OP, I had two children who seemed to dislike being babies. Both high needs, grumpy, poor sleepers. They were both massively frustrated and became much happier when they could walk. But I was as certain as certain could be that there were no physical reasons for their grumpiness. In fact I knew that it was frustration because if they were upright and held outwards and allowed to interact with the world constantly, they were fine. Or as good as they got until they were more mobile.
I think I'd be tempted to go to the GP with as much info as possible. When the crying happens, if anything at all helps, exactly how she moves when crying. Because things like silent reflux can be hard to diagnose but there are solutions out there which involve more than just soldiering on.
Having an unhappy baby is a tough, uphill slog. It does NOT last forever. I promise. Whether your baby has some reason for it which is identified or if they're just struggling with an existential crisis which will resolve with the application of time, there will be an end to it.