Here are my few cents:
No GMAC doesn't give partial credit.. you have to get all 3 right to get the question as correct.. even if one subquestion is wrong.. its WRONG..
Regarding attacking IR questions.. try not to do all the questions if you are getting all wrong.. its better to focus on 6-7 and get 4-5 right than to get nothing right.. as you will get more and more rights you will grow in confidence and realize that IR is nothing but what you actually know.. its a mix of Quant and verbal.. that you already know.. look from this positive perspective and you will easily conquer IR.. also keep a track on which type of IR questions are easier for you and which are difficult..
Like for me multi source seem tough.. so i skip them altogether.. and do the rest 9 questions in 30 min and get atleast 7-8 rights... n it works for me most of the time..
Do let me know if i can be more helpful!!
Bye.. Have a happy new Year!!
PS: Please consider kudos if you found it helpful