Your second scenario. You sew the left bodice front fashion fabric to left side underlining fabric to finish the front edge. Press and fold underlining back so wrong sides are together. Repeat for the right front bodice pieces. It will look like a finished front along the center front opening edge of both left and right sides. Then baste the two layers together (the left front and right front), with serging if desired around the un-sewn edges.

I do not understand this part very well:

[–> My favorite modern shortcut technique is finishing the center front (or back) edge during the flatlining stage. It is, however, based on period construction. The center front edge is finished (sewn) *before* the underlining is attached to the fabric.

Right sides together, sew the fashion fabric center front to the underlining. Do this for both left and right sides. Press flat, grade the seam, clip at the curves, turn and press the seam flat.]

Do you sew the the two fabric fronts first together and then to the underlining ?

Or

Do you sew with an ordinary stitch (not the serger) the fabric fashion fabric front to the underlining each? So you have two pieces of the front (fabric and underlining)? Then using the serger to finish the seam?

Or do you mean something else :-)? I do not understand it completely probably because English is not my native language. Sorry for this.

Yes, you could cut open the dart take-up and press it open to reduce bulk in the layers of the fashion fabric and underlining.

If the canvas isn’t too heavy it could be used for the bodice panels, but not for the sleeves. I’d recommend cotton organdy for the sleeves; or even a couple layers of poly organza would work for short budget. The muslin *might* work in the sleeves depending on your fashion fabric. Layer some swatches to see how your interlinings and fashion fabrics work together.

You do NOT need hair canvas. But using stiff organdy is a nice alternative for tailored collars and cuffs. And yes, avoid any fusible materials/interfacings. As another alternative, use the muslin but starch it well. Dip starch is best for stiffness, but a spray will work too (and easier to find).

Can i use it in place of hair canvas for tailoring needs? Or do i ABSOLUTELY need hair for tailoring? (Im REALLY trying to avoid fusible as ive heard horror stories and hair is hugely expensive for me, and i want to wash when i can, so canvas is looking the like the best option for me)

Is it historically accurate? Is it accurate for the lower end of the working class? Im really trying to not spend a ton of money on the inter layers, but i want to look to the best of my abilities.