You only need a secondary mold if you ant to preserve your first plaster cast. If this is not needed, you can simply add plaster on your original cast to sculpt hair and clothing etc... just soak it in water first, as fresh plaster will not bond to dry plaster.
If you must preserve your original casting , you must indeed make a reproduction and make a secondary mold which to cast your repro.
Alginate is not the best product to do that. An alginate mold will be fairly easily removed from a live model but will most likely be ripped apart and damaged by a hard plaster shape or break off parts of your plaster original when removed. (example: live ears are soft and flexible, they glide out of the alginate mold. Hard palster ears will not come out without distroying the alginate or beraking off or both. Same with large breasts, fingers etc...)
If you original is a fairly simple shape, you can attempt to make an alginate mold on the plaster. (I do not think any sealant is needed.)
But In most cases, this secondary mold must be made in silicone rubber. This is far more resistant than alginate and very stretchy. You only need a thin layer than can easily (but carefully) be peeled off the plaster original fine details. You still need a plaster shell on top of it. But the shell must not adhere to the silicone skin (no cotton fibers needed ) If the shape is complex. You may have to build this shell in several parts. Use straight plaster for this, not plaster bandages. (you have plenty of time to work, your model is not a live person...)
Only problem: Silicone is fairly expensive. There are many different sorts of silicone molding products: tin cured, urethane, pourable, brushable. All have different properties: hardness, strechability, color, shrink ratio. Some require a release agent, some dont. some stick to itself, some dont, some a prone to air bubbles, some are not...
The best is to talk to your supplier. They should be able to advise you on the best product for your project.
If you know that you will need several casts from your original mold, it may be a good idea to make your original mold in silicone instead of alginate. There are a number of skin safe grade of silicone. One is BodyDouble, another one Plastil Gel 10.
I hope this helps.