I would assume that "expected duration of the entire project" is a single number. Or are 27 and 33 intermediate calculation steps? Might be fine. Nasty correlations in the paths.
A4: If your expected duration is a bit less than 34 weeks, the probability should be close to 50%, not 8%. I would expect an evaluation of a gaussian distribution or something similar here.
How can you save 8.5 weeks if D and F are done in parallel and save 4.5/4 weeks each?4.5 weeks for Task D and 4 weeks for Task F for a total time saving of 8.5 weeks.
Bonus question: Add 2 weeks planning time, how you can you crash the tasks to save this time?