It is my understanding that tyres should be changed at a maximum of 6 years, irrespective of tread. However, Felix42's spare, IF it has been 'kept in the dark' would be OK as a 'get you home' tyre.
Re balancing weights:- depending on the out of balance forces,to properly balance a wheel might need weights on the inside or outside of the rim, so worrying about unsightly weights on the outside, might not produce a properly balanced wheel.
