Interesting find about the nayru's being lower. I always jump out of shield into nayru's anyways, and i guess it's coz the move doesnt auto cancel upon land.
however, as for it being a viable edgeguard, i highly doubt it.
here are the stats
Nayru's Love
Total: 59
Invincible: 4-11
Hit: 12, 15, 18, 21, 24, 27,
Reflects: 4-41
The time from the last hit of nayru's which does the most damage at 27, ends more than halfway before the move does. That means the lag time after the last shard disappears is more than double of what it took for all the shard to appear. Meaning no matter how fast you were, getting back to the ledge would probably require an upB.
More importantly, the hitbox of nayru's is directly horizontal. You can't hit opponents recovering diagonally or from below. This means jumping off the ledge is a bad idea anyways with nayru's, because it won't connect with most opponents.