Correction: it appears that you do get the Proof of a Concord Kept reward when invaders die while spawning in and from the environment, at least sometimes. It just doesn't show the "item picked up" message and takes some amount of time to add the actual item to the inventory. I had this happen to me: spawned in, got the "duty fulfilled" message and was sent back home before I managed to even finish the "spawning in" animation. Got the "duty fulfilled" message, never was notified of a Proof being added, checked my inventory to find I still only had 4 Proofs. Switched to Mound-Makers and had a similar issue, but when I checked my inventory found an unexplained 5th Proof. Needs more testing.