It's been a long time since I worked on a Charger, but here goes:
Yes, I'm pretty sure the rear valance has to be out of the way for removing the corners.
Yes, the order would be corners, valance, quarter patches.
If you have good corners they'd be worth saving & reusing. They're the key part in getting that area as it should be.
I've made new ones in the past, but I've seen really nice repops in the last couple years. I haven't actually worked with them, but they look very nice.
You have to carefully test fit all the parts and get them properly positioned relative to each other before welding the corner in.
What you're seeing on that corner to quarter seam is seam sealer... on some cars it was a neat tidy job and others it oozed out, giving that raised string look.
Yes, you can reuse the rear valance... yes, reweld the drilled out spot welds.
The rear valance repro's that I've used were pretty nice and fit well.
Hope that helps,
Rick