Low risk
Cracked paint is either a paint-film problem, such as a thick coat or matt over silk, or a movement crack underneath. Which one it is decides the repair.
Paint that peels off new plaster has not bonded to it. Do not paint over the failed areas: scrape back, clear the dust, let the plaster dry and start the mist coat again.
Caution
Most paint smell fades in days. If a smell is still strong after that, or smells sour, work out whether it is slow-drying paint, old paint or something else. If it smells of gas, leave and ring 0800 111 999 (0800 002 001 in Northern Ireland).
Vinyl and painted wallpaper block water and steam, so the surface has to be scored first. Steam in short bursts, and do not soak plaster that is already soft.
Describe the job and get a fixed price from a local painter and decorator.