So the reason why the Ghosts (Heroes) did not spawn is indeed a bug. Pretty much the sum of your encounters (Worms 25 + Ghosts 60) is exactly the cost of the enemy (85). Just a minor mistake in the script that ignores such a combination, gonna fix that eventually. Don’t know about the Nemesis not spawning, I could not reproduce that, unfortunately.
As for the shard, don’t know which one that is, but you can check the end of this post, all their locations are listed there: [Mod] Nydiamar
EDIT: By the way, these Worms (Aetherial) are way too cheap, gotta up their price.
EDIT2: I take it back. This is not the reason, and the sum can be equal the cost.
EDIT3: I figured it out. I think Grim Dawn has some sort of limit on how many heroes, bosses and so on can spawn in an area — this means if I would spawn a bunch of heroes, and then again, and again, after a while I would stop being able to spawn any in. You can notice this inside the arena, where the first few encounters always got some heroes, while the ones later on have none or only very rarely one.
This is why the Ghosts (Heroes) encounter stops spawning after a while, it only consists of heroes, so it can’t spawn anything.
The same goes for the Nemesis encounter, I assume. It also did stop spawning anything, even after leaving the map and doing other stuff for a while and then coming back.
I honestly can not think of any solution right now. The only thing to make this special arena and mechanic function would be to limit the amount of tries to maybe three or four per game session, and then also decrease the entrance fee for picking up the loot.