In my opinion the trusses will be the cheapest place to get the height. Get creative. Go with a 6 or 8/12 pitch. Then you will have room to do a scissor or tray on the inside to get your extra height. I think the higher pitches make the building look better too. My house has an 8/12 so I built...
I am doing mine with the 2x4s on the inside of the post. As stated it is easier than attaching between the posts and you only lose 1 1/2" of floor space. As for using blown insulation in the wall it is fine as long as you can access the top of the wall to add insulation as it settles.
Dimensions of the garage would be helpful. Mine is 40 feet wide. I framed a man door on each side leaving a 32 ft opening. I put up a double cannonball track and built 4-9 ft sliding doors(2 on each track). I can slide the doors to have a 18ft (or so) opening either in the middle or one side or...
My vote is also for the 10 ft doors. You will never wish the doors were shorter but may kick yourself in the behind if you need an extra couple of inches and do not have it.