The 14th Amendment is pretty all-inclusive. It says you are guilty even if you just "give aid or comfort" to the actual insurrectionists. Trump tweeted out that he loved them and they were special people, after the insurrection. That sure sounds like "aid and comfort" to me. Even more so if he pardons any of them.
Lots of Democratic pols are now talking about the 14th Amendment. I think if anything gets to federal court, the judges will decide the 14th applies.
As for proof in court, it may be difficult to prove Trump incited it, but a slam-dunk to prove he failed in his duty to end it. He had a legal obligation to protect Congress, and he just watched it on TV and blocked attempts to help them. As a result, people died. Once again: "Aid or comfort".