They can certainly sort of flicker in and out of existence before disappearing entirely! That's totally fine.
As for the second part, not really. This part of the event is more about actual physical reality than perception. So if two people are looking at a tree and the tree disappears, the tree is gone in an objective sense. Hmmm...though actually, in terms of things like flickering in and out of existence? In that case, one person might see the tree disappear and the other might not.
Once the tree is completely gone then it's gone for everyone, though.
LMFAO that was way over-complicated, sorry. TL;DR yes but only at the time when the thing is flickering in and out of existence, after that the thing is gone-gone.
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As for the second part, not really. This part of the event is more about actual physical reality than perception. So if two people are looking at a tree and the tree disappears, the tree is gone in an objective sense. Hmmm...though actually, in terms of things like flickering in and out of existence? In that case, one person might see the tree disappear and the other might not.
Once the tree is completely gone then it's gone for everyone, though.
LMFAO that was way over-complicated, sorry. TL;DR yes but only at the time when the thing is flickering in and out of existence, after that the thing is gone-gone.