"Eternally youthful duchess's elegance some recalled" - ageless.
"...wrong when defending manuscript writer sent in" = amiss. Something to do with (Martin/Kingsley) Amis? (This clue 'ran on' from the previous one but I don't think that's important in this case.)
"Buddhist pries goes up to school for mixture" = amalgam. I'm not getting the 'gam'.