Hypothetically speaking, if one or more players at the end of the year were, for example, 50 ranking points from the having the #1 ranking after the World Tour Finals, could they play in a subsequent ATP challenger event such as Helsinki (I think a win there is worth 125 points and presuming it would become one of the player's best 6 results for 500's and under) to attempt to capture the #1 year end ranking? Or are commitment players (top 30) ineligible for Challenger events?
They're allowed to play any event. If they score enough to have the result into the Best 6, it will be counted. I doubt they'll play though even in those conditions. They're crying for time off, nothing would compromise that.