FWIW I think the highlight from the window is showing a curve that makes it look like the whole ceiling is bowing quite scarily.
We have some reed ceilings here that look in a similar state - previous occupants put embossed wall paper over them to cover the cracks. When I painted some of them there was no indication of anything loose or hollow sounding and some of them have rooms above we use regularly - I reckon that the reed offers a tighter grip on the plaster than the lath elsewhere in the house but I'm no expert.
I don't really understand the issue with the chandelier? I assume that as well as fixing it to a joist or two you want a decorative moulding as well - wouldn't that be attached to the joists as well not the plaster?
If it doesn't move and doesn't sound hollow then I'd be tempted to keep it. Can you get a look at it from above?
We have some reed ceilings here that look in a similar state - previous occupants put embossed wall paper over them to cover the cracks. When I painted some of them there was no indication of anything loose or hollow sounding and some of them have rooms above we use regularly - I reckon that the reed offers a tighter grip on the plaster than the lath elsewhere in the house but I'm no expert.
I don't really understand the issue with the chandelier? I assume that as well as fixing it to a joist or two you want a decorative moulding as well - wouldn't that be attached to the joists as well not the plaster?
If it doesn't move and doesn't sound hollow then I'd be tempted to keep it. Can you get a look at it from above?