I'd be inclined to make a wire or foam holder for the collar so that deterioration wouldn't be an issue. Look up "Elizabethan ruffs" for examples.
For the skirt, I think a petticoat and / or hoop skirt would be a good solution. If you don't want it to show under the skirt, you could build in pockets on the "petals" and run the boning down inside them. You could even affix that darker grey layer to the bottom/underside of the hoop itself and hide it that way.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan