i had a similar situation (though it had even less of the back legs) with one of my second instar p.paradoxa, for some reason (deffinitely not chewed off by locust/crickets) it only had the front four legs. it was not very stable (picking up the container caused it to swing back and forth as it was only hanging from the middle two legs) but it could still catch fruit flies as long as they happened to walk near him (couldnt really chase them well at all). it managed to eat and moult like the rest of them just fine, and after moulting had regrown the back legs (although they were smaller and did not have any lobes, and it did not really use them much, but still). anyway just thought i would mention that i have seen mantids survive fine with worse (but similar/related) injuries, i'm glad yours did too.