It sounds like your chooks have a lovely life and you really care about them. That's great to see.
Hopefully I can help with my opinions to answer your questions.
So my questions are:
1) if chooks are free range with access to scratch and the odd treat can they be malnourished?
2) what are the visible signs (if any) of malnourished chooks?
3) at 22 weeks, is it ok to start feeding layer pellets (despite them not starting to lay yet)
4) can one chook be malnourished when all the others seem ok? why would this be?
5) if she is malnourished, what is the best way to proceed?
thanks for any advice!
1) Yes, free range chooks can be malnourished. I would be surprised at this time of year because they would have just been through a time of abundant fruiit, bugs and plant growth (depending on your local climate and weather conditions of course). But from now on, especially over winter, they are likely to really struggle to find enough food to keep them plump and warm.
2) Malnourished chooks may show no visible signs of malnutrition. Feathers cover a lot of sins, and I have discovered that I can't tell anything about their underlying body structure just by looking at them. My chooks look really fat at the moment because the weather is turning colder and they are fluffing themselves up more.
3) It's definitely fine to give them layers' pellets at 22 weeks. I think this is exactly what they need - layers' pellets available all day so they can eat as much as they need. If they don't need them, they won't eat them. Free-range chooks get plenty of exercise so there's no danger of them eating too much, and at that age there's no danger of too much calcium accumulating in their systems before they start to lay.
4) I think it's possible that all your chooks are a bit underweight. They would all benefit from having layers' pellets available. The divided breast you described earlier is not a sign of malnutrition - it's more a feather growth pattern.
5) The best way to proceed is to give them layer pellets available any time they want to eat them, but especially morning and evening, so their crops are full before they go to bed. There are all sorts of boosting suggestions on this site - like feeding cat food, mince, bandsaw dust (waste material from butchers) etc etc. All those are options, but a good staple diet of pellets is the first step.
If you can pick them up and feel their condition (and/or weigh them) that's good, but you would need to feel a normal-weight bird to know what you're feeling for. With them roosting in trees at night, catching them is likely to be a mission! I say just feed them up.
I will also add that I think the advice you received about not letting them eat grass is really odd. My chickens free-range all day and eat a vast amount of grass. They have a balanced feed available all day (in my case a grain-based one rather than a pellet-based one) but they choose to graze on grass most of the day. Perhaps the vet was meaning cut grass? Some people keep their chickens penned then throw in cut grass from mowing the lawn and this can cause problems. Chickens picking and eating grass of their own accord is a whole different thing and should be encouraged.