I think her conformation is pretty good! Looks sturdy, well put together, and confo wise, there isn't a setback. Beautiful TWH!
Now, about the bit, I don't really think it's a problem. If she responds 'well' in that bit, there is probably a smooth bit out there she'll go with, too. I don't like chain mouthpieces, but as you stated, OP, your trainers have her there for a reason. I know tons of Walkers that have to have that kind of bit or they'll do nothing but get in that fancy pace all over the place.
I know a man down here that rides his at a walk and a pace thing (Forgot the name) and has to keep his in a harsh(er) bit because he's a runaway without that reminder. Your trainer may have you in this bit with her to ensure you can control her.
But if she's calm, sweet, and a lovely mount, go for her! She seems like a petty neat mare! :)
Cheyenne, QH Paint Trail Mare. Happy Trails, Y'all! Twister, Appaloosa Barrel Horse. Turn 'n Burn! These 2 horses stole my Heart!