You might need 2 packs. Probably not but I wouldn't risk it. If you get 2 boxes (often they are 2 for £7 or 2 for £8 at Boots or Superdrug) then you can do a little strand test and then if it works for you there will be plenty left for a root touch up. Whilst you can't keep mixed colour, remember that it is equal parts so you can mix a tablespoon of each bottle together and do a touch up or a strand test or whatever and reseal the bottles and use the rest another time.
Whack it on your roots, give it 20 minutes then run the rest through your hair, I think. Then rinse. I think you will get a fairly even result then without looking morticia.
I have just done my roots today with that colour, and I gave it 20 minutes on my roots, wet the body of my hair (ignore that bit, I was being ultra cautious about it not "taking" too much on my ends) and literally 5 minutes on the ends. But I'm not trying to "colour match" root to tip here so you might need 10 minutes on dry hair and that is what you should do.
Its a bloody lovely colour and I don't see me changing it any time soon and I have been every colour under the sun trust me.
Section it off, yes. Use a brush to brush it on if you can handle the thought - it is much better if you get used to doing it that way, then you can avoid colour build up in the future. Those women you see with way too dark hair who can't do anything about it and can't even get highlights because they will be dark ginger are women who have been home dying too dark colours and running them through each time. Don't be that woman.
Will it stain everything? It will stain a porous surface (grout, lino, natural stone) but not a sealed artificial surface (bath should be fine - careful of tile grout, most tiles it shouldn't)
Ellangirl - if you want to risk £8 to get your lino clean, you could buy ColourB4 by Scott Cornwall, mix an equal amount of each part together and paint it on your lino, leave it for half an hour, see if it gets it off. Its hydrasulphate so shouldn't damage your lino, and is designed to reverse the colour process. If your lino is a deep colour do a patch test first in the corner where the bin is. Or just don't bother 