Most airline programmes do not even have a retroactive period .. and certainly those who do only have a 30 day retroactive period. Etihad is certainly generous with its 90 days.

Your failure to sign up for Etihad's FFP within the 90 days (now that's 3 months) of your flights comes down to your own fault.

At this time, I wouldn't be threatening Etihad's FFP programme with anything. But perhaps you could 'lie' and said that you did sign up for the programme before you flew, and you just cannot understand why your miles have not been credited. Blame Royal Mail as per always .. that you did send off the application form and you did inform Etihad etc.

Perhaps that's the way to do it as Etihad currently has no other FFP that they are part of. And even if there are, you haven't even joined up to their FFP (which may not even offer retroactive flights) to even transfer miles over.

Otherwise, look at it as a lesson learned and sign up for any programme in whatever airline you fly next time.

And welcome to Flyer Talk!