P1) Everything that exists has a cause
Just one cause?
I think that if we ask for, or point out, a cause, we are making a selection. Perhaps we are drawing attention to a connection that isn't immediately obvious. For example; 'The moon causes the tides
But we are not saying that it is the only cause; the tides are also caused because the temperature means the sea water is liquid, or because a body collided with the earth (the debris forming the moon), or because of gravity, or the conservation of energy and so on. A full explanation for 'What causes the tides?
' would have to include everything in the universe. And of course everything in the universe would also be an 'effect'.
Thus, the cause of everything would be everything.
Another way of putting it would be to point out that 'everything' is not the name of a thing. It is just a placeholder for particular things. A particular thing, like the tides, has causes, but an 'everything' does not.
So I do not think P1 is a proposition. It has no truth value.