The intended meaning is that 'it', described simply, is 'everything'.
Would I write:
It is in short...everything.
Or:
It is, in short...everthing.
Instinctively, I believe that a comma is needed before "in short".
But the confusion arises here:
Is it a common enough phrase that the comma is not needed? Is it a publisher's preference situation? Author's preference? I've seen the phrase 'in short' both with and without commas. Normally two commas are used, as it's a parenthetical, but the ellipsis makes the second one unnecessary.