Well, I tried soaking Ringlet in boiling water, and the neck is still on tight. DX I got her tail out and gave her the same treatment as Raincurl. I also used Q-tips to reach in and clean out the rust in the body and back legs, which worked extremely well.
I shone a bright light under the ponies to illuminate any dark splotches inside. The majority of the rust was concentrated around the body, and there isn’t much in the head or front legs, from what I can see. The body/back leg rust has now been cleaned out, so they’re in pretty good shape! I couldn’t find any more washer pieces after a lot of shaking, prodding, and rinsing out. I did a pretty thorough sweep.