1. First of all is lining the wall with lining paper neccessary? The paper is going onto a plasterboard wall which was drylined and skimmed say 5 or 6 years ago, no cracks or anything, just a few nibs from where it's been emulsioned (easily taken care of with sandpaper I would think).
It might be painted with contract matt or vinyl silk/soft sheen..not good surfaces to paper over..definitely size the walls first and allow them to dry..if the moisture in the paste soaks through the contact matt you could end up with bad bubbles..
2. If lining paper is needed, can I do it vertically if I stagger joints with the finish wallpaper? It's a long room and I'm on my own so I reckon despite more cuts the vertical hanging would be easier (if needed) than cross lining. Also do I need to size the wall before lining?
Taking my last comments on board..I would rather line the wall first..its the ideal surface to paper over...you can indeed stagger the joints..just start at the opposite end you intend to start papering...but double check first just in case any of the joints do overlap..and yes..you might still need to size.
3. As the finish wallpaper is paste the wall first, any good tips on how not to get paste on the front of the finish wallpaper? I imagine as you are brushing bubbles out the bristles will come in contact with the paste on the wall and then back onto the front of the paper.
Dont apply too much paste to the wall...use a shortish pile roller..or a medium woven one..
Dont use a artex caulker or anything hard to smooth the paper out..it will squidge the paste out of the edges...
Keep your tools (especially your papering brush) clean and free from paste
If you dont have a decent papering brush use a long pile roller sleeve..
Keep a bucket of warm water and a sponge to hand and wipe the paper over after you hang each piece..whether you think you have got paste on it or not..better still, keep your brush or long pile sleeve in water and use it wet..(not dripping) when you are smoothing the paper out..it kills two birds with one stone..
Keep your hands clean and free from paste..
Dont roll the whole wall with paste in one go...do about 600mm at a time..enough for each length,
Cut your lengths first and back roll them so you can just drop them down the wall..