It’s complicated to explain but the easiest summary is “neoconservative ideology” and a certain kind of postmodern imperialism that came to dominate US elite culture following 9/11. Read up on PNAC if you want to dig into it more.

To some extent but it’s really not that important. Oil gets traded for all kinds of things besides dollars, it’s never been so big a deal that the US would engineer a very dumb war over it. Maybe there were some for whom it was a motivating factor in their support for attacking Iraq, but by and large the animating force was not concern about what currency Iraqi oil exports are traded for. It makes no real difference to anything, it’s not significant.

Linking the invasion of Iraq with their selling oil in Euros makes no sense at all.