Snareandracket: O.K. I accept your invitation. This is to aid in understanding "how to reason" with Jehovah's Witnesses. Have to warn you that people say I think differently. I have been laughed at regarding my conclusions, but usually I can laugh with them. Just had that happen at a witnesses stand not to long ago.
I would first suggest you consider the legnth of time a person has been a witness. 1987 would be a dividing line. That was the last specific statement that the creative days are 7,000 year long. The January 2014 Awake article on creation is vague on the length of the creative days (Alan F. critique 1/2014 Awake).
Younger witnesses are given the "intellectual freedom" to imagine the creative days as thousands of years long. They can imagine enough time for life on the planet to have been brought about by God thru his Holy Spirit. "All things are possible with God"
Older witnesses, pre 1987, are having to conform there thoughts to a specific length of time. That is 50,000 years. However the last 1,000 years are for Christ's reign so the window is reduced to 49,000.
So in my opinion an important part of discussing, evolution versus Watchtower teachings, is the matter of how old is life on earth? When a witness is faced with contradictions, the fall back position is, "But let God be found true, though every man be found a liar" Rom. 3:4. So your approach to ask those willing to discuss is right on target. Your audience needs to be open to the discussion first.
Next in my way of thinking is to view the origen of the universe. To this day you can find a similar divide among young and old witnesses. Steady State vs Big Bang. This is where I am laughed at by witnesses. When I state that I believe God used the big bang to create the universe it invokes out load laughter. I even believe I can use the Watchtower literature to prove that is the case. Even more laughter follows that statement. At least people are haveing a good time.
Anyway, back to your point of how to reason on science/evolution. Perhaps start the discussion not as evolution vs. the Bible. Rather the discussion would be: What does the Bible teach about the origen of life?