Did your design files come in different formats? You will need software to convert them to the correct format for the machine.
There may have been issues with the design digitizing which lead to the problems. The thread may have been threaded wrong. So many other possibilities. Visit the dealer again and see another design.
Yes, to turning in all the original accessories that came with the machine. You keep any extra ones that you purchased. Check the manual as there should be a list. Keep a copy for you and one for the dealer so you know they were all turned in at trade-in.
See if there are any dealer events coming up that you can attend. You get to see the machine brand in action for a whole day. There are usually discounted prices and packages available.
Don't be shy about negotiating discounts for the other brand machines. They are available too. If there are upgrades to the machine, you can negotiate to have those included. These large machines need a rolling bag so be sure to inquire about them. Good luck on your decision!