I'm so sorry these things are putting a dampener on your planning experience - it should be a fun, exciting time!
I would have a chat with your family members that you want to come. Take some time and call them up, thank them for the gifts of money, then explain that you feel that they are close family members and how much you would love for them to be there - maybe they have a genuine reason, such as ill health, or mobility? 3 years is a long time for circumstances to change.
If they don't appear to have any good reason, I would try to calmly explain that it is hurtful that they made the effort for your cousin and not for you. If they still don't come, then try to accept it and enjoy the day with people who have made the effort. But definitely try and find out why they can't come first.
Regarding the dress, that is a tough one. It was lovely of your friend to try and make it, but it sounds like she had a completely different idea to what you had in your own mind. Have you tried the dress on? As many dresses look better on a person than on the hanger.
If you have and you still don't like it, is it possible to ask her to help you alter it to make it more like what you had in mind? I think a degree of honesty is the best policy - don't tell her you hate the dress, but just that in real life it didn't work on you as it did in your imagination.
If it's not possible to alter, then I would definitely wear the back up dress - it's your wedding, and you don't want to look back on your photos and regret the dress.
I wouldn't tell her that your mum wanted to buy your dress (unless she did), maybe just say that after comparing the two you decided to go down the more traditional route after all.
Hope this helps!