Wow!   Good thinking.  You've really considered the process and the potential pitfalls.
I will just offer some thoughts: -
if you are considering making a negative mould, why not do it on a real person who has all the necessary body parts!
To make a laminated form in such a size, it would have to be quite strong before removing it.  Do an experiment over something smaller (wide bowl or something) with layers of bond (office) paper and kraft (brown packaging) paper interlayered.  My guess is that you might need twelve layers for the strength you need.  It wouldn't be too bulky.
For removal from the mannekin (if you do it that way), make align marks across where you are going to cut.  Cut carefully all around.  Once you are sure it will release, stick a few strips of fabric across the join on one side to act as hinges.  Remove and join the whole.
Of course, people often begin freeform, with chicken netting.
If you have a hollow body form, I would not advise layering inside because it is more likely to lose shape as the moisture is absorbed.  If you have access, you could take cardboard tubes, or make squared hollow rods and fix them in from one side to the other.  In this way, you could form quite a strong internal structure.
I'm sure someone else will add their own twopenn'th
Good luck
DavidO

Hi!  I'm new here, but not new to PM.  I thought I would contribute some thoughts.

First, I would cover the mannekin with SEVERAL layers of plastic to protect it (I am assuming that it is fabric).  Then I would cover the whole thing with PM.  I would think that 3 layers would be enough to make a form rigid enough to remove, as long as it is completely dry when you remove it.

For cutting, a pair of bandage scissors is wonderful!  They are fairly small, with a lip on the tip of the bottom blade that can slide under bandages (or PM) without puncturing or cutting the victim.

You might even be able to cut straight down the front of the piece, then bend it along the spine to remove it from the mannekin.  Once it's free, tape it back into the proper form.

A clever way to make proportionate arms and legs:  Lay someone down (they have to be the right size) on a large piece of paper and trace the outline of their arm or leg (in the position you want).  Separately, make outlines of the other dimensions of their limbs (showing proper thickness).  You may want to make separate outlines for each major bone on these outlines, such as forearm, upper arm, thigh, lower leg, foot, hand, etc.  SAVE THE ORIGNAL OUTLINES.  Use the patterns to trace onto cardboard.  Then either use opposite slots (half a slot on one piece, half a slot on the other like you find in cases of wine bottles) to turn the flat drawings into 3D arms & legs, OR cut them in half lengthwise and hot-glue them in place.  If you are using slots, I would use tape or something to make sure they stay in place, as sometimes they tend to try to flatten out.

Now crumple pieces of paper to fill the Vs of the limbs to make them rounded, and cover with PM.  Attach to torso with more PM.

If you add a bit of PM "skin" to the connecting end of the body piece (marking where the real connection is, so you don't make it too short), you can make slits on the extra (which I would make as long as 6"/15cm for thigh connections, which must provide support), then use the strips of extra "skin" to attach to the torso, then applying several additional layers of PM for strength.

TIP:  if you find you need to waterproof PM for some reason, use real shellac, which is thinned with denatured alcohol (aka "shellac thinner").  Apply two layers very thinly, so you don't soak the fragile paper to the point of collapsing.

Sue