1. What would be the best fabric to use for the lining?
Coutil is what "real" corsets are made out of, you could use a heavy twill, stiff and non stretcy
2. I'm hoping to make a reversible corset, and was thinking about having a double-lining in between the two outside layers of fabric. I was planning on sewing in boning channels straight into the fabric, instead of using the boning casings and sticking the boning between the 2 lining layers. Would that be a good idea, or should I do something different?
I think I would flat line the fashion fabrics to the lining fabrics, first and sew the channels in all layers (put the bones through the two lining layers.
3. I know steel and spiral steel boning is the best to use, and just wanted to double-check something I've heard in the past: Flat steel boning is used next to the grommeting and for the busk, and spiral steel is used for all other channels, correct? Please let me know if a different way is better!
That sounds good, I think it depends on how stiff you want the corset to be and how curved the casings are. I have made corsets with all steel bones and ones with all spiral. It kind of depends on what you want I don't think there is a "better" way than what you want/works for your project.
4. What is the best technique for sewing on the biased tape/finishing for the upper and lower edges for a corset? The last bridal-style corset that I made I used ribbon and it did not come out as smooth as I would have liked. Any tricks or suggestions to make it come out nice and flat?
Smooth how? Ribbon is not bias and doesn't curve as nicely. I think what I have done in the past is stitch and turn the sides and bottom and put piping along the top. Of course piping and bias are a lot a like. I think if you use bias it will be flatter(?) than a ribbon.