Glad to hear that she's eating well and gaining weight, even though she's growing slowly and as you pointed out she may be a bantam. If so then they only gain a few grams per day in weight to begin with.
I'm glad you had a good result with the chick food mashed with water. A good way you can gauge your chick's progress is to weigh her every day. Just use some digital kitchen scales that measure in 1g increments. If she isn't gaining weight then feed her the mash more often.
Some chicks also really love the regular chick starter with a bit of warm water added to make a paste. That's all my lone chick, Pancake, would eat until he was 3-4 weeks old!