Broken needles are a huge red flag. Are you using too small of a needle? I usually use a 16 topstitch. Are you moving the machine or quilt too fast? If you're moving quickly and the needle is small, it's possible the needle is bending in the fabric and hitting the needle plate.
Thread nests on the back or bottom of the quilt usually means issues with the top thread. Take it completely off the machine and re-thread, making sure it's threaded correctly and nothing is impeding the thread path.