Hi.
Linseed oil just makes the pulp easier to work and tougher when dry.
How to sand papier mache? Take some sandpaper, rub it against papier mache 
The finish you want will be achieved by using very very fine pulp, gesso, and then sanding.
I'm using nothing but toilet paper at the moment. I've found that the really cheap stuff is the best for a smooth finish. Also, you don't need to waste half your life tearing it, just put it all in a saucepan and boil it. Give it a stir occasionally and that's all. A real time saver! Wallpaper paste and plenty of glue makes it really smooth if you apply it carefully.

I haven't tried any other oil, no. I'm not a great one for experimenting and prefer to follow the advice of others here. I'm sure I read somewhere that if you bake your PM, the linseed is meant to make it waterproof?
What is gesso? it's basically a primer.
It doesn't matter when you sand, as long as the piece is properly dry! Some irregularities will be obvious, others will show up when you're painting. You're best off sanding & priming, sanding & priming until you're happy. Paint can be quite thick of course, so you might want to sand your paintwork down too. And then tou might want to add a couple of coats of varnish, sand, and varnish again!
Bear in mind that you can come up with some lovely paint effects by sanding through one coat to expose the colour underneath.

No, though I do have the remains of wire wool I was thinking of adding.

I've used fiberglass fibers making plant pots with peat moss, cement and sand, but sometimes you have to singe any fibers on the surface which you couldn't do with PM.  Fiberglass tape - the drywall kind - might work, too.  I'll let you know.

:idea: Gesso is a thickened, strengthened paint used to obtain an ultra smooth surface.  The traditional gesso is used in gilding and is made from ground chalk in a rabbit skin glue.  You only need it if you want an ultra smooth surface.  It can be applied in layers, each sanded.  The PM surface itself is interesting and textured so you can simply paint this.

It's good to see someone experimenting.  Their results can inform us all.  (Go on Davey, be a devil :twisted: )
Linseed oil doesn't really make it waterproof.  As Davey said, it simply helps smoothness and may improve hardness slightly.  As you said, it can stink.  I once visited a linoleum factory.  The smell was one of the worst I have ever encountered.
DavidO

Hi, your cats sound wonderful and I'm sure we'd all like to see them when you've finished. The image you've given of all the different sizes puts me in mind of a cat's chorus with them all gathered together.

Sorry I can't help you with the 'blitzing', but thought I might throw in carving to the conversation. I sometimes carve my pulp pieces when they are dry to give them more precise detailing. All you need is a little whittling knife or possibly a small veg knife.

My mix has been kind of nubby.  I put it in the blender wet.  Next time I'll try it dry.  I'm keeping an eye out for a used food processor which would have a larger container.  This is probably a dumb question, but are you supposed to dry the pulp for storage?  I made snowballs and put them in the freezer.  Ignorance is bliss.   Today I'm working on the bowl I had in my head.  Strips of fiberglass for reinforcement seem to be working well so far.  Here's my first ever project, a kitty inspired by my own Rosie.  I decided to leave her faceless so anyone with a calico could see their cat in her.

I drizzled white glue over the whole business after I had them placed and smoothed it out.  If it ever dries I'll put another layer of PM on.  My goal is to have the bowl turn out looking like pottery.