Just came across your images while looking for photos of grass-carrying wasp sign... I don't know if you felt your questions were answered completely, but if you extracted this wasp larva from its cocoon, it must have been about ready to pupate, which would account for its stillness.

To clarify, have you ever found one of these cocoons in the absence of grass? And have you found a cocoon that you knew to be Exyra? This link shows a cocoon of another Exyra species, described as being pure white, and it looks fairly loose, unlike your cocoon, which looks like the tough, brown, parchment-like material typical of wasp cocoons.

As for the frass, I'm not sure what the frass of one of these wasp larvae would look like, but perhaps you can assess how it compares with the frass you normally see associated with the moth larvae.