post 2 updated! I've gem skipped through the exchange goals. It won't give you everything (well not for me) to finish the carriage. The next step is for Thurston to go on the trade and time and resources are double.
You DO NOT have to do the exchange with the girls, but you should do as many as you can because it's CHEAPER IN TIME AND RESOURCES to get the parts for the carriage.
The Wedding Alicorn should be placed NOW as it drops parts for the carriage.