2) When people jab a finger - the finger is usually index finger or middle finger?
What does "jab a finger" mean in "she jabbed a finger Raymond-ward."?
Usually it's the index finger. In this example she closed her fist and pointed her index finger in the direction of Raymond. Sometimes the wrist moves a bit up and down (once/twice etc) at the same time as the finger is pointed.
3)What does "a head" and "dry" mean in "I'm paying sixty quid a head for a dry roast turkey dinner"?
"a head" means per person. It is costing 60 quid for each person (per person) for the meal. A 'dry roast turkey dinner' means the turkey was cooked without oil or water .... it was cooked dry in the oven - by heat and air only, I guess.
4) What does "downwind" mean in "He smoked as we walked. I tried to position myself downwind of the noxious toxins."?
'downwind' is usually the direction the wind is blowing. If something is 'downwind' the wind is blowing towards it. The sentence you mention is a bit confusing because if he were downwind of the smoker, he would be getting the smoke ! I can only think the writer meant the person wanted to always walk 'in front' of the smoke. Far enough in that 'downwind' direction (direction the wind is blowing towards) that the smoke would dissipate sufficiently.
5) What does "whereby" mean in "after some initial awkwardness whereby sammy had no idea why a pudgy hand had been thrust in his face"?
'whereby' means 'in which' or simply 'where'.
6) What does "walking frame" mean in "he gets confident with the walking frame."?
A 'walking frame' is a walking aid that old people use to help them walk. It's more supportive than a simple walking cane. A walking frame is usually gripped with 2 hands, and it usually has 4 'legs' that provide more balance to the ground.