Maybe stop thinking about "perfect week" for a while.
Is she at home with you (as opposed to at preschool)?
If so, then once an hour (or whatever interval seems right)
take her to the toilet and sit her down.
I like that you've got her cleaning up after herself.
However . . . this may, in fact, be the pay-off.
I'm guessing that maybe she feels like a grown-up
because she has to wash her panties in the sink.
What if, instead, you have a bucket of sudsy water
(+ whatever, Lysol or something) in the bathroom
and the panties just go into the bucket,
for you to deal with later on.
Your assumption that it may be "more in her head"
than a real physical problem may get in the way
of solving the larger problem.
With all that fiber and miralax and fruit, she should,
I think, be having a (relatively) solid formed BM regularly.
You said "potty", not toilet.
Is she using a little potty chair or the big people's toilet?
If she's on the big people's toilet, is there a step-stool
or something like that where she can put her feet?
If her feet are dangling without any support,
it is more difficult to push out a BM.
She may have some resistance to having a BM
in the potty or on the big toilet for any number of reasons.
I think before you can reward her for clean panties,
you need to re-teach her to have her BMs only in the appropriate place.
Good luck.
S.