It's not that the shard destruction is what causes Rejoinings.

From what we know, Ascians first imbalance the aether in a reflection, which cause the "dimensional wall" between Source and Shard to fissure, with some aether from the shard trickling back to the Source. Then, they trigger a calamity on the Source which, in order to stabilize itself, just suck all the available aether from the nearest source : the already weakened Shard, absorbing it altogether and de facto destroying it.
Except if the Source is actually just another shard the Ascians picked at random, I suppose the whole Sundering thing make it that it's the "base" and it only flow that way.

So I guess you can destroy it as much as you want, it'll just be a barren rock in the end, unlike a shard where its dimension just collapses and goes back home.

If you actually manage to destroy the planet and all its aether completely, I suppose the shards would just be there since the planet already took quite big hits through all the calamity without really affecting the other non-imabalanced reflections.

Perhaps it's Hydaelyn triggering the Rejoinings in a last resort to stabilize the Source? In which case, she may choose to just break down a random shard to try to fix it until there's none left.



As for other planets, we have 3 occurrences of aliens using Aether : Ultima (High Seraph, not the fancy metalwork Nero made), Midgardsormr, and Omega. That last one literally is a machine able to manipulate aether, so its creators must have at least known about the stuff.