re: Columbus and JEM
Indeed, only two shuttle flights are absolutely indispensable to honor international agreements: those delivering the Columbus and the JEM.
From 28 flights, reducing to 20, or 18 or 11 or even 2 shuttle flights...well, things evolve.
But there is one payload that should be considered the most important one. Unfortunately, it's the last one and it might never leave the ground (unless an ELV can deliver it): the Cupola.
If the ISS is to ever completely change its purpose and be transfered to the private sector, a room with a view becomes a necessity!
Raoul