Can I remove ceiling joists without risking the integrity of my roof system?
I believe that you're correct about the ceiling joists being entirely for cosmetic purposes. The rafter tie keeps the walls from spreading, and it appears to be adequately suspended by the roof structure as a whole. 

You're probably doing the rafter tie a favor by eliminating the ceiling joists from its bearing, and unless you add substantial weight back onto it I don't have cause for concern. 

All that said, I'm sitting here in Minnesota looking at your place in Washington through a peephole. You really do need to get a qualified professional on site to make a more reliable assessment of the situation.