This is edging close to "debate me!" territory.
In Scrooged, it seems that ghosts are solid unless they choose not to be, since the GoCP passes through a door and Frank's boss pushes his arm through a plate glass window, but gets ambushed by Frank and repeatedly shot. Or you can just handwave it all away as being supernatural stuff that isn't really relevant to the plot. Frank's office is magically put back together again after his first visitation, and as the GoCP says about visits to the past, present and future, "it's not live, it's like a rerun".
Donald Bellisario had a rule regarding Quantum Leap, which was not to examine the fine points of minor details too closely if they don't actually impact on the story or characters. This was largely an excuse to get away with the nonsensical time travel element of the series, which enabled the strong human drama elements. If you're focussing on how time travel works in the series, you're looking at the wrong thing. It's there as a convenience to facilitate the rest of the concept, and is not meant to be scrutinised so closely.
It's the same with a Force ghost suddenly able to interact with its environment. You're not expected to examine it in such forensic detail, and if you are, you're losing focus on what the film wants you to notice.