How difficult/possible/necessary is it to protect the traverse across the ledges? I assume no rock pro, but guessing some ice screws in the right conditions would suffice?

Also,
does anyone have any info on the steam caves in the crater? Are they safe to access?

The rock on the gib ledges route is horrible, crumbly. The ledges themselves are pretty big, I think only an epileptic would need to place pro there. I did the route in Feb and don't recall any good ice near the ledges proper. There's a little chute at the end of the ledges that would take pickets or possibly some screws depending on conditions.
I think the time required to move through that area roped and placing pro is potentially hazardous. We got off route at the beginning and ended up on the ledges a little later than we wanted - even in the winter there was a lot of rockfall. Just my two cents but I'd recommend hauling ass whenever you are near rock on Rainier.
Don't have any info on the steam vents.

Been through them twice, both times no protection. But, did use a running belay with pickets one time on the chute that is just after the ledges that goes up to the top of Gebralter Rock.