Is there even proof Webb supported Trump in 2016.
All people point to that one video where he said he may
Webb's public statement was that he wouldn't vote for Clinton, but he might vote for Trump. After the election, he made some positive statements regarding the incoming President, but did not confirm that he voted for him.
I believe that Webb voted for Trump in the privacy of the voting booth, but wishes to preserve plausible deniability. He took the posture that FL Panhandle Rep Bob Sikes (D-Crestview) took in t968, when he said, "I'm voting the Democratic ticket, but I'm not asking anyone else to." Webb bolted (I suspect) and voted for Trump, but he never asked anyone else to.
Remember that before Jim Webb was a Democrat, Jim Webb was a Republican who worked in the Reagan Administration. It's not like he has some "I didn't leave the Democratic Party; the Democratic Party left me" story to tell.
With hindsight, it appears Webb was largely just a generic Southern Republican who was especially pissed off about the Iraq War putting his son in harm's way, and that incited him to switch parties and run for Senate. Then, he decided he didn't like being in a party headed by Barack Obama, so he retired after one term and retreated from politics until 2016.