My theory about why Bolton went missing (or made himself scarce and unavailable) recently is actually probably good news in one sense; It means he doesn’t really have any good meat against Trump, but his problem is that his book isn’t released until March and he needs to keep interest up. If he holds a press conference like Graham wants and he offers up nothing-burger, then his prospects for selling a lot of books in March go way down.
The bad side to this is that the dems will use it to try to draw out and extend impeachment, even though Bolton wont give them jack-schiff once he is under oath.
And Trump assuredly has evidence to discredit any lies Bolton would even think of telling the Senate.