This is a great request. I don't think five question would be enough, but if I had an opportunity to ask five questions, I would ask the following.
First, what made him travel with Lennie all these years? Surviving in a brutal world by yourself is hard enough; what enabled him to stay with Lennie?
Second, since George stayed with Lennie, I would ask what benefit did Lennie provide? Was it friendship, companionship, or was it something else?
Third, why didn't he run away with Lennie at the end? From the beginning of the book, it seemed that they had a plan if things did not work out well. Why didn't he stick to it? Was it fear or something else?
Fourth, as a follow up to the previous question, what was his rationale for taking Carlson's gun and shooting Lennie? Was it to spare Lennie from a lynching that would definitely happen? I'd like to hear it from George's perspective. In Of Mice and Men no answer is given. We have an act without explanation.
Finally, now that Lennie is dead, how is George able to move on?