I’ve repeated the experiment several times. The echoes worked every time.
In fact, there is one more thing I discovered: The hypothesis (expressed here, for instance) that excessive damage would ‘go through’, as it were, and kill despite a revive echo can likely be discarded. Observe as I take 125 damage while I’m at 31 health in this example, and I am restored to 162 (which is within rounding distance of 322/2):
The excess damage is not being applied to my post-revive health, which I take to mean that the entire hit is ignored. I suppose it’s still possible that damage exceeding 31+162 would oneshot me through the echo. I’ll see about clipping a counter-example if I can.
(The echoes could really do with having their heart icon restored, or have an obvious visual effect, and like many others I believe they should grant a second’s worth of invulnerability.)