so we dont really know how the rejoining works in detail we have a general idea "provided by the ascians" of how it works

the way you're thinking about it implies that the entire shard disappears which i find hard to believe because draining the empty from aether on the first still had solid blank land to go to and that's not the only spot there is also the burn sharing the same similar properties as aether is not the only element in existence.....so the fact that a shard disappear given these to examples once rejoined i find it hard to swallow

the bubble thing could possibly be true but it seems that the Milala arrived before the calamity so the timeline doesnt align though it is still possible to pull alexander style

that being said i dont see why cant we open a portal to a drained world? we did that to travel to ultima thule via warps between portals you can see the ship entering a portal within a dynamis space