Perhaps nice compact solutions simply don't exist if your expressions are too complicated. Do you really require an algebraic expression? If not, plug in your expressions for w and v, then use Solve/NSolve.
1) Can you find the displacement (with respect to the flagpole) of each runner at any given time? When they meet, what must their displacement be?
2) How long does it take for the train to stop with the given acceleration? Thus how far would it have travelled?
The purpose of the second comb is to find out the polarity of the charges! You have no way of knowing if you're using just a single comb. You shouldn't expect to find them charged in their natural state.