Taking Q2 first:
If the couple own the property as 'joint tenants' then neither of them individually owns even the smallest share of the house. It is their legal partnership which owns the whole of the house. (The difference between the two individuals and their 'partnership' is akin to the difference between a person running a limited company and the company itself. They are separate legal entities).
So, when one of the partners dies, no part of the house forms any part of his/her estate. It belongs to the partnership, which now has one surviving member. So that person now owns the whole of the house.
However if the couple had owned their house as 'tenants in common' then the deceased person would have owned a distinct proportion (usually a half) of the house, which would then form part of his/her estate and count towards the 'cut-off figure' you've referred to.
Now Q1:
Rules here:
http://www.hmcourts-s...te/why_will.htm#chart
Lastly, Q3:
Anyone is free to give their own money (or their right to receive such money) to whomever they like, at any time.
Chris