Point 1, there is absolutely no way, not unless you created a vacuum in the cavity like a double glazed window, that the air in the cavity has any thermal quality that is even worth considering as a benefit. In U-value calculations, even when using partial fill insulation (i.e. 50mm insulation...
What you'll notice is that you don't need your heating on as much because the house will stay warmer for longer. Loft and wall insulation will make a difference that you will notice.
They can inject the wall insulation from the inside if you have simple painted walls and therefore easier to...