They are different species. Sweetsop is Annona squamosa: small, round, pale green, with a bumpy skin that pulls apart into sweet custardy segments each holding one black seed. Soursop is Annona muricata: much larger, dark green, covered in soft spines, with fibrous white flesh and a tart, almost citrus-and-cream flavour. The names sound like a matched pair, which is exactly why they get swapped. Soursop is also the Annona that carries the strongest published epidemiological link to atypical parkinsonism through heavy habitual consumption of its fruit and leaf teas.

Yes, treat them as toxic. Annona seeds concentrate annonaceous acetogenins, and powdered sweetsop seed has been used traditionally as an insecticide and as a treatment for head lice. That folk use is precisely where the danger comes from: seed powder or oil contacting the eye is a documented cause of severe corneal injury. Eat the flesh, spit the seeds out whole, and never blend, crush or grind them.

No. It grades D for total FODMAPs and D for excess fructose with C for polyols, so it is a moderate to high FODMAP fruit at a realistic serving. A couple of segments may be fine for someone past the elimination phase, but the whole fruit is a large fructose and polyol load and the density of sugar in it is easy to underestimate because the texture is so soft.

Sweetsop and sugar apple are two names for the same species, Annona squamosa, which is why their grades here are identical. Custard apple is the ambiguous one: in India and Australia it usually means Annona squamosa, but in the Americas and Europe it is often used for cherimoya, and cherimoya grades a full step higher on FODMAPs and polyols. If a label or menu says custard apple, look at the fruit rather than the word.