Figured it out! To edit an existing title already in the timeline, click the "generated media" icon (filmstrip with square on top) embedded at the extreme-right of the text-lane in the timeline. (Why simply double-clicking the desired text-lane isn't assigned to this function, I don't know.)
Usability issue when expanding the timeline: Also, when you click the "+" symbol to expand the timeline horizontally, why the does the timeline not anchor itself to the left (you have to keep manually scrolling left to see your populated lanes)? As wonderful as Vegas is, these seem like two glaring usability issues (the latter one in particular).