I don't understand why its "fishy". Andrew himself said after the news of the engagement that they were consulting diaries to find a date that worked for everybody (harder given they chose to marry on a Friday). Obviously they found a date but Camilla already had an event and chose not to cancel an event booked in before the wedding to attend. At the end of the day I doubt Camilla will be crying she is missing it and will send the couple her best wishes and love and Jack and Eugenie will have 800+ other people there to focus on, I'm sure when trying to find a date it was more important Andrew, Sarah, Beatrice, the Queen, Philip, Charles, Edward & Anne could attend rather than Camilla.
Remember William missed Peter & Autumns wedding because he had a prior commitment and that was for his own cousin not just a niece-in-law. I really don't see the fuss and any gossip of anything more untoward is just people trying to create a drama and push an agenda.
On a side note I wonder if they chose the Friday to avoid marrying on Sat 13th Oct.