Hello! Welcome to the board!

1. For most PVA/white glue, half water and half glue should do.  Some cheap brands are more watery, so I would use less water.

If you're using Methyl Cellulose: For every cup (UK, CAN or US) of adhesive that you think you'll need, allow 3.4 grams or 0.12 oz or 2 tsp (US/UK/metric) for every 1 cup of water. SPECIFICALLY, divide the water in half. Heat the first half of the water to almost boiling, sprinkle on the full amount of MC, and mix VERY WELL; then add the other half of COLD water, and mix well. If, for some reason, it appears cloudy, let it set overnight for full absorption of liquid.

The ready MC should look and feel like raw egg whites or honey. If it's thinner than you want, add a bit more MC; if it's not thin enough, add a bit of water and mix well. All of it will stick.

If you know you'll need a larger amount, like a litre/quart, follow the above instructions, but use a litre/quart of water (again, divided into half hot/half cold), and mix in one-half ounce (2 tablespoons) or 11 grams (2 tablespoons) of MC.

2. I would say you'll need at least three layers, and maybe five. Let the first three dry well, and then push on it a bit and see how sturdy it feels.  Using either of the adhesives, there is no problem gluing more layers onto dry ones.

3. Most emulsion housepaints are latex/acrylic, and most primers are, also, so I don't see any reason to use both. I doubt that you can get away with just one coat. It never seems to look "right".

BUT -- never try to put an OIL-BASED primer on anything, and then try to apply a latex/acrylic coating -- it won't stick properly.  If you use an oil-based primer, all coatings afterward MUST be oil-based.  If you use a latex/acrylic base (emulsion) base, the top coating can be either latex/acrylic or oil-based.

Sue

Sounds good . . and a lot of fun.  Don't worry about the salt.  As long as your first layers are dry and firm you can lay successfully on top with the recipe Sue has given you.  Copy paper is a bit firm and you will probably see the edges but I guess you're not aiming for a perfect smooth finish.  It can always be sanded before or during the painting process.
I'd suggest aiming for about twelve layers then see how firm that is for your purposes.  Dry it thoroughly between every two to three layers.  On the size of strips, bear in mind that the paper will shrink as it dries so don't use large sheets (which wouldn't smooth over the contours anyway).  Strips of about 1" x 6" should work well but you've probably got the feel of it as you work.
If you want wrinkled texture you could use toilet type paper and manipulate it into the ridges that you want, squeezing it firmly into place like clay.  This would be great round the eyes and mouth for example.

Also, methyl cellulose seems to dry faster than PVC, if time is important. 

I wouldn't worry about the salt, but I wouldn't put it in if you do other projects.

I've started using torn brown paper bags, and like the results for strength.

Sue

How fun! 

You're working fairly large scale, so I'd say the size of your strips would in part be determined by the complexity of the shape underneath.  So for around the eyes you'd need smaller strips, and the sides/top of the head you could get away with larger ones.

My technique is a bit different.  I don't pre-tear my strips very often ... instead I paste up (with methyl cellulose + pva ) layers of larger sheets of newsprint and copy paper on a serving tray (cookie pan would also work) using a brush or wallpaper roller to spread a thin layer of paste between the paper layers. I also crisis cross and alternate the papers as I'm layering them which helps increase strength. 

so:
* paste empty tray,
* 1/4 sheet of newspaper,
* paste,
* sideways copy paper sheets overlapping (with paste in the overlap) to equal the size of the newspaper,
* paste,
* another 1/4 sheet of newspaper,
* paste.

Then I the tear the size of strip I need ... And since I'm tearing wet paper it's fairly easy.  If I do 3 layers of these 3-ply strips ... I'm actually putting 9 layers on my piece. 

I also haven't found much of a difference between pieces where I've let them dry between layers or just done all 3 layers at once.  So i do as many as I have energy for and stop when I'm tired.  The caveat there is that because I'm initially pasting them on a flat surface, the layers are fairly well laminated to start with.

I do also do a fairly thorough rubdown with my fingers/hands to make sure everything is also well laminated once it's on the form. 

I've used 'strips' as big as 4" x12" or larger if I can get away with it.  : )

You said it's a 4' piece ... I'm assuming it's decoration, vs. something for the kids to climb on, yes?

Yes it's definitely decorative, not something to climb on!! I've tied fishing line loops through the chicken wire in 6 places so I can hang it from the ceiling. That's the other thing, I want to keep it as light as possible so the removable Command hooks for the ceiling don't fall, another reason to try to minimise the number of layers.

I've just done 2-3 layers of copy paper on the nose tonight so I'll see how firm it is when it dries, and decide if I need to/have time for another layer. The pre-pasting trick is amazing though, I'll definitely try that out!

I originally planned to do a top layer of kitchen paper to give it a bumpy, almost scaly texture but instead, I know it's poor practice but I didn't wrap paper around the chicken wire armature to smooth it out, and the hexagonal indentations look like big scales! I was lucky there!

There's one more fun detail I've added- I taped coloured LED Christmas lights inside near the eyes! Once it's all dry I'm planning on cutting the paper mache away from the eyes and pasting one neat, thin layer of paper over it. The cord comes out the back. Once the party is over, I assume I can cut a hole in the back of the paper mache, cut off some of the chicken wire, and get access to the inside without any damage? Can you cut through the paper mache layers with a craft knife?

Thanks again for all the advice. Something tells me this won't be my last foray into papier-mâché!

Brown packaging paper should work fine. If you change colors for each layer, you should get more complete coverage.

Use a box-cutter knife if you have one.  They're easier to hold onto, and stronger.

If you can put the piece out in the wind or in front of a fan, it will dry faster.

It sounds as though your Dad was a 'Pom' because in UK stanley knife is a basic bit of DIY toolware.  It even has a keyhole saw attachment, which I find very useful.
It may be too late for another tip but I'll include it in case it helps anyone else researching this type of work.  You can get interesting texture by moistening string (or even rope) with neat PVA and laying it on the surface, then applying paper over it.  As Nives (Nives Cicin-Sain) says, "If you want a valley you must have two hills".  So if you want a deep groove, lay two lines of string/rope then paper over them, building up to the outer level.  If you know anyone who does upholstering, they may have scrap lengths of piping, which is excellent for this purpose.