Bird not recovering from Sour Crop

Hi,
I just thought I should post and ask for people's thoughts. I have a girl, 2 1/2 years old - she's an ISA so arguably reaching the end of her life and I've been trying and failing all week to cure her sour crop. Wondering if people think I should put her down to save her distress.
That said she doesn't actually seem in distress - though we all know how good they can be at hiding that - but she has lost a lot of weight and is sometimes a bit lethargic.

Before people ask, here's what I've tried to do to treat the sour crop.
* Noticed on Saturday she was being docile and found the watery Crop
* For the next couple of days "tea-potted" her in the morning to drain the foul fluid and pipetted olive oil to her. Kept her separate from the other girls to isolate her from their normal feed. Didn't restrict her access to grass though because we don't have a grass-less area.
* Made sure throughout the time she only had access to water with ACV in it. Made sure there is an ample supply of grit including trying to hand feed it to her.
* By Tuesday she still hadn't improved so swapped to isolating her in our broody cage to deny her access to grass and fed her live yoghurt.
* Haven't emptied her out since then as once we limited her to just water and yoghurt it seemed to go through her okay.
* Last night she seemed a lot better so allowed her to have some pellets before bed.

However this morning when I went to let them out I felt her crop and she still had food in there.

What we were doing with isolating her and what felt like starving her seemed so cruel - even if it was for her own welfare - so today have left her in with the others to sort herself out and see if that does to improve the situation.

I'm open for suggestions/criticism on if we have been treating her wrong but I'm not sure what else we can do.

Any thoughts appreciated

Well the new girls are the closest to non-commercial hens I could get my hands on locally - a light Sussex, a speckled Rock and a copper Maran.
Hopefully they'll have more success in their old age than the ISAs have had.

Yeah, neither of my (remaining) ISAs laid beyond their first year. Which is fine, they're pets not a production facility - I just don't want her to suffer.
The thing was yesterday she had really perked up. Almost back to her old self again. She was wandering around with the others and generally being a happy looking chicken again. But this morning she is refusing to move and back to having a watery crop again. Worse than that her feathers are dirty and her comb is now drooping.

Never had to dispatch a chicken before - does anyone have any recommendations - in preparation a while back I got some dispatch pliers but have never used them. Time to research how to use them I guess.

If you haven't tried it get some live maggots now. We had an upsetting time with this last year - two vet visits.  No need to separate from others etc.

Yes.
They didn't think there was anything they could do for her and eventually put her to sleep