Annamich
First of all I wish you and your Fiancé much happiness and congratulations on your engagement!
Next I just don't understand the people who are being so rude to you on this thread! You have done nothing wrong! Far from it! You have been trying to help people and have had one nasty person deliberately destroy the kind sacrifices you made for the sake of other people. It is clear that you tried very hard to arrange your Wedding Date so that it would be as easy as possible for the two families. You took into consideration the expense that it entails for them and the travel and time off which would be a holiday.
When your F's cousin said she was getting married in 2019, as you said:
"they booked for 2019. Fiance and I decided that two close family weddings would be a huge strain on the family (being greek we’re quite extra with our outfits and very generous with pinning money on the bride and groom so weddings are a huge expense) so we would postpone for the summer of the following year."
That is extremely kind and thoughtful of you! To postpone your wedding for a year so it would not clash with another family member's wedding and cause financial strain on everyone, is very kind and generous of you both! I am so annoyed with people who say nasty things to you when you have given so much thought for others concerning what should be a day all about you.
The evil person here is the "Hijacker Cousin" who, on finding out you had decided to leave 2019 free for her wedding, decided to have her wedding the same year as yours; 2020!
Considering she was not engaged when you initially announced your plans, and that her mother was the one asking what your date was, it is obvious to me that this girl, on hearing that you were having a wedding, decided straight away to eclipse you. She said she was getting married the same year. (2019 at first). Then you did your extremely kind and thoughtful act and said she should have 2019 and you would wait till 2020 (to give the family plenty of time to recover from one wedding and prepare for the next), she Miss Spitefull, "I'm-going-to-out-do-you" straight away deliberately changed her wedding date so it would clash with yours!
Why can't people see that she is a wicked and nasty piece of work who has twice upset your wedding plans when you have simply tried to help everyone and put their needs before your own?
The truth is, this little bitch will always do something nasty to spoil your wedding no matter what you do. It is her personality. She has lied to everybody and has set her "flying monkeys" against you. I am really sorry, but you are up against a very spiteful character here and the less you have to do with her the better. I hate to say it, but I think you would be better off avoiding her and anyone in her "camp". I would, though write and explain that you had postponed your wedding from 2019 to 2020 because you later learned HC was getting married in 2019 and that it would be rather a strain for the families to have to deal with two weddings close together. I would say that now the -evil troublemaker- cousin has suddenly changed her date to the be near yours, it is not possible to change yours again. I would say somehow what you said to us:
" I feel like all our family has somehow made us feel guilty and inconsiderate when in actuality we’re being shafted."
perhaps ...."We had decided upon our Wedding in 2019 without any knowledge of HC's plans. Then HC said she was having her wedding in 2019 too, we reluctantly decided for the sake of the family to postpone our wedding until 2020. We cannot be held responsible for the fact that, after we did this to help the family, FC then decided to change her wedding date to the same year as ours. (It seems she is determined to have her wedding the same year as ours!" I would write all that as well as what I wrote above, really spelling it out. Send it to all the family who'd be invited, normally. Then arrange a dream wedding somewhere they can't come.
Despite it being your dream, would it be possible to have an alternative dream wedding somewhere where she cannot find you and will not know that your wedding is happening? Have you ever dreamed of a beach wedding for example?
My cousin's daughter and her husband were married secretly in a small chapel in the north of Scotland. It was sad for her mother, but their family had been disrupted by some terrible events and arguments (not even about the wedding) mainly because of one person's personality which was selfish and nasty. She and her husband were not prepared to have a wedding where this person and people he had on his side could spoil anything. I believe they cherish the special day they had in the peaceful surroundings of a beautiful place that was very dear to their hearts.
I really do feel so sorry for you. This HC is a very nasty and tricky person. I really would think of how you can have a lovely wedding without her there. Or even her family if that is possible. You might change your plans a lot but you could tailor your day to a place and ceremony that is very special to you and your husband and designed with true meaning for the love you have and does not need lots of agonising about how to look after the rest of the guests.
Good luck! Lots of love! Remember, it's about you and your dear Fiancé, being married and together because you love each other! That is wonderful! Don't let anything else come before that! 