1) He doesn't fit the bill of a traitor
I wouldn't be too concerned about the definition of a traitor here
Right now it's a Saboteur more than anything rn
But if it turns out to be an ally then it'll be a traitor
2)How he reached there - yes he is invisible but egghead is well guarded as well
That is a fair point as Bonney, Straw Hats, and CP0 ran into trouble reaching the island
I'm thinking the Clear Clear fruit is what gets him through, like we saw in Thriller Bark the Clear Clear fruit can make other things invisible as well. Which could easily extend to making a ship invisible also. And we saw that Vegapunk's sensors were potentially not able to detect him, and when Absalom used it even Chopper couldn't smell him anymore.
So it would simply be a matter of making a small but viable craft invisible and sailing right in
3)The explosion - not shiryu style
I think the explosion might have been an unrelated cut, though it will be hard to say for sure until we get an update on the fella
If he got blown to pieces the explosion was related, if he simply disappears than it wasn't
Makes me realize that Lilith has been missing also, she's currently the red herring for me rn so I think she got nabbed as well