Answer to answer 1. So is it possible to keep adding meeples to Leipzig just by adding pieces to the same road?
You can send up to 4 meeples to Leipzig, one per quarter. Each quarter provides bonus points for a particular feature in the base game. You will be able to send a meeple to Carcassonne each time you complete a road conncted directly to Leipzig or whenever you complete a road connected to Leipzig through on or more crossroads with villages.
Here is an example where
Red can send a meeple to Leipzig from a road connected to Leipzig through 2 crossroads with villages.

Answer to answer 2: ok, let's assume there is one meeple on the Wainwrights quarter. Would the green player get 6 points?
Green would score 1 point bonus per tile on the road just completed, so
Green scores 2 points for the road (2 tiles x 1 point/road tile) and 2 points for the Wainwrights quarter bonus (2 tiles x 1 bonus point/tile). All in all,
Green scores 4 points this turn.