ovidgsd It would appear you might be over designing the foundation/girders, joists and walls. Nothing wrong with over designing or over building; it is preferable to under designing and placing your fate in the hands of 'good luck'.
Piers/posts: a PT 6x6 on the concrete pier will support your load with no problem at all.
NOTE: Some of the below may depend on whether or not your soil can support your structure on just 6 piers. In some places more piers would be required to ensure their ability to carry the weight of the structure and contents.
Girders: With beams being built up from three layers of 2x12's you can dispense with the center beam. One of those triple laminate 2x12 down each side would work with a pier spacing of around 7 feet. A slight overhang at each end coupled with the 12 inch diameter concrete piers makes that easy to achieve. The girders can be built from shorter lengths as long as the joints are over a pier.
The girders do not need to have the ends 'boxed'. However, the joists that sit on the girders would have rim joists nailed to the ends of the joists to create a joist 'box'.
On a building with trusses or rafters and a gable roof the building weight is carried by the sidewalls (parallel to the roof ridge) and the end gable walls simply support themselves. Usually the center beam can be eliminated in a structure under 20 feet wide, as long as the joists are sized correctly and the two girders are adequately supported.
Joists: Southern yellow pine, 2x10, grade #2 can span the full 16 foot width without a center beam; 16 " OC, 40# live load, 10 lbs dead load; even good for ceramic tile. Twelve inches OC would be stiffer, but not really needed unless you wanted it. See note below on insulation.
I'm not clear on what you mean by "Will do 2x10 blocking every 12" between joists."
A row of solid blocking down the center, parallel to the beams/girders. would be sufficient. With a center beam it is mandatory (IRC).
Walls: According to the IRC 16" centers with 2x6's is all that's required, even 12 foot tall. Twelve inch spacing will make it more difficult to insulate with products such as precut batts. Twelve inch centers also reduces the effective R-value as there are more studs per wall length and studs are less efficient insulators. Winds over 100 mph or a location subject to special seismic loads may require following some special rules.
A double plate at the floor in unnecessary unless you have some special requirement because of some special thick flooring or something else unusual. Double upper plates make it easier to tie the wall corners together and allow for more versatility in rafter or truss spacing.
Question regarding the loft: Are you planning the loft floor to be at the 12 foot wall height level?
Exterior wall sheathing: The structural layer could be OSB or plywood, 7/16" or 1/2" or thicker. Then the weather resistant layer (building felt or house wrap) and then the cosmetic layer (lap siding, panels, stucco, etc.)