Yes, definitely silk organza. Maybe even two layers if the charmeuse needs it.

To clarify, you baste the underlining and skirt fabric together along the long vertical edges but don’t stitch/baste these layers along the waist and hem edges. Basting those edges can come later just before you finish the hem and/or waistband.

For sagging fabric you could let the fabric hang to naturally stretch it before cutting your pieces. Depends on your fabric type/weave/content and what you’re trying to make for how to flatline the skirt panels. If the fashion fabric is very heavy, it may not be a good choice for the skirt style you are attempting.

You don’t need to flatline the skirt. I would recommend, however, a deep hem facing (see this post), and fold down a good amount at the top to support the pleats. Check out my post here on cartridge pleating. For silhouette support – use petticoats! 🙂

Many skirts in this era are not fully flatlined. Winter wear they might be. I would recommend a deep hem facing; it’s historically appropriate from the era and will help hang the skirt properly over your petticoat layers. (So glad to hear you’re wearing them!) Basic cotton or organdy are both good options. Play with layers of both fashion and hem facing fabrics to see how the they behave together. Good luck!