Hi there,
I have just used petroleum jelly as a releasing agent, and not had any problems - I've just made 12 masks from 2 moulds for a play! I have heard that some mask makers soak their plaster casts in water if they have been stored for some time to stop them being too brittle.
Personally I would be dubious about completely sealing the plaster cast as then it can't "breathe". Any moisture still in the mould would be sealed in. I'm not sure if this would present problems, but I haven't been making masks long enough to have experienced any problems. The people who have taught me don't seal their casts.
As far as positive and negative go, the easiest explaination is like this: If you buy a plain plastic mask from a shop, the positive side is the side of the mask you decorate, the negative side is the side that goes over your face. If you light the negative side(or inside) correctly it can look like a positive (or outside).
When I make a mask, I use the plaster cast of my face (unsealed) and place it on some clay on a board. This is so as to raise the level of my 'face' so that I can mould round the side of my head if required.
I then use clay over the cast of my face and mould the clay into the shape I want. (Currently I am making a set of Commedia masks).
Once this clay 'positive' is to my satisfaction, I build a wall of clay around the mask - it doesn't need to be more than 3inches (8 cm) high, except at the nose (usually the highest point of the mask) where your clay needs to be higher than the nose.
Cover any bits of exposed plaster with Petroleum Jelly to prevent the next lot of plaster sticking to it, or cover it with a layer of clay.
Mix your plaster of paris and pour a thin layer over your clay mould ensuring the plaster flows into all the details. Continue stirring your plaster until it starts to thicken and then pour over your mould - you will need to get your hands in and shape it so you have a sort of cone shape completely covering your mould. IMPORTANT! Make sure you have a flat top, (this will be your base!) The plaster hardens quite quickly, so you have to move pretty fast.
Once the plaster has set and cooled, turn it over and remove the clay and the head cast. Carefully remove the clay from your mould - it will tend to stick in places and throw it away. (you can salvage it, but you have to soak it for 3 days then dry it and it's not that expensive!)
You now have a negative mould. Be careful not to scratch the inside of this mould. When making a mask, coat the inside of the mould with a layer of petroleum jelly as a releasing agent, if you don't you'll never get your mask out!
You then do your papier mache as normal - I use blotting or construction paper with a slightly watered down PVA glue. You can press your papier mache into the mould and because it takes the shape of the mould, it should require less finishing. 3-4 layers is a good amount. Ensure you create an overlap at the top of the mould to allow easier removal of the mask.
Once the papier mache is done, (and you can put this away if you run out of time and come back to it as you are layering on the inside) set it aside to dry for at least 24 hours. The wetter your glue mix, the longer you need to leave it.
Once it is dry, gently ease the mask out of the mould, this will take time and patience and does depend on the complexity of the mask. Once it is out, you just need to paint and sand as usual, cut out the eye holes and cut around the edges. A Scalpel or craft knife is preferable to scissors.
Hope that helps. If not there is a step by step guide in Agostino Dessi's book (?35 or approx $62) or you can find on-line guides.
With the handle issue - I've not come across anyone selling these. You could always mould some in clay and then cast them in a resin, that way you get exactly what you want. Other than that I've no idea.
Sorry for the length of this reply! It's not meant to be a novel!
Good luck.
Alan C
P.S. If I ever get round to taking some photos I'll put some pictures up.