DE does not "treat" mites or lice. (It's nice, natural, might help prevent in a clean coop or added to the sand for "dirt baths", etc. -- might help prevent, will not treat.)
May I suggest that if you saw mites on her before, and then you dusted her with DE... check again, carefully.. under the wings ("armpits") and on and around the vent.
If she has mites.
I suggest Sevin Dust or "Garden and Poultry Dust", repeat in ten days.
If she had been in the flock, or if (when) your flock gets mites, dusting each chicken, as well as cleaned coop, nesting boxes, roosts, etc. Repeating in ten days.
If she has been quarantined in a cage... after she is clear of mites, can you moved the cage into the run (and the coop at night)? Let her be exposed to the flock, with the cage walls between them, with her safe in the cage. Put the food for her and for the flock up against the cage wall, so they can eat "together". Give it at least a few days that way. (With a fence or cage wall between them, with them eating head to head, "together".)
I absolutely agree with twocrowsranch, and think five weeks is plenty long and keep an eye on them.
Is she the only Bantam hen? Does she look like the other birds in your flock. Chickens tend to pick on someone who's different looking. And, all chickens have to find their place in the pecking order, you can't prevent some jockying for position in the flock (bickering).
Clearly you are doing everything you can; and doing it well and thoroughly. Good luck with introducing her.