Thanks for your information and insight.I wonder if Jimmy and Jonesy felt it would be inappropriate for them to attend,given the way they rarely socialized outside of recording and touring.They are both famously reticent,and English to boot.They may have genuinely thought that Robert would prefer having just his family and closest friends there,and not the whole of Led Zeppelin and the media circus which that might cause.Additionally,we know that Jimmy was in a mess by this time.Perhaps it felt like too much to deal with.In any event, given that it seems pretty certain that Robert wanted them there,it's very sad that they didn't go,and this matter seems to me to be a rarely mentioned "elephant in the living room" when the frictions that have arisen between them are discussed.
It is so heartening that in spite of all of this,on top of the whole "don't forget my phone number next time" episode,thousands of other things I'll never know about and a combined age of about 224 (including Jason),they've managed to stun the world again.