Hey Chaffy,
1) Yes you can both work together in the same blacksmith shop, though I think the deed will be in one name, you could marry and be part of the same bloodline, that way you could in essence both own it.
2) You will be able to obtain the land for your shop during Exposition (however this will not be any time soon so don't rush). You will need to use Exposition points to purchase the land and get set up, which I believe you will also need the pledge package - Bloodline, Settler or Proprietor (only one of you needs it though).
3) This feature I haven't looked into before, but I would hazard a guess the deed belongs to your family, so as long as you still have heirs to spark your soul into you will own it.
4) Very much so, everyone will need them, may I ask what server you plan on playing on? I may be able to help point you in the direction of a guild that you could work with.