The limo has to do with his trust of Trump. He was not getting into a limo that Trump might have control over. He knew by then Trump was going to furious and want vengeance.
I believe Pence heard the chants. they were loud and he is not deaf.
Yet, he refused to leave the Capitol when the Secret Service urged him to. Your belief that Pence heard the chants is not evidence. If he did hear the chants and still refused to leave, he must not have been too fearful.
It is a huge stretch to suggest the limo was an attempt to harm him.
If a crowd outside the White House chanted "Hang Donald Trump" would that be a criminal threat?