Carcinisation is a bit more of an adaption to environment (convergent evolution into same-ish shape for the by-chanceb best utility of it ... ie a perfect body) vs niche mimicry which is in relation to another specific species (ie some else's body).

It's more that the lobster plan (long body) is really quite good in many niches, but the crab plan (wide body, no exposed tail) works better in more productive ecosystems that have more predators. So anything lobster shaped coming up from the deep mud will have to reduce its tail or get sniped by a fish

Proper ants only have 6 legs, though. But yeah, these spiders-turned-to-ants would have 8 legs.

Well, and crabs technically have 10 legs, with their foremost pair typically equipped with pincers. 🙃