Line tool is definitely planned for both roads and walls. It's just a bit on the back burner right now. It'll basically work by selecting point A then either selecting point B or dragging to it. (So, pretty much what you're describing!).
As for the road auto-upgrading, I'd like to add this in as well, but from a interface standpoint I'm not sure how. It would be very simple to add a "Upgrade until maximum" flag, but in the future that won't be an ideal solution, because someday more complex/expensive roads will be added (The next one above the current max will require iron for example). So if players went and assigned roads to be "upgraded to max" it would get very expensive, very fast.
What I'd like to do is an "Upgrade until <Road type selected>". But I haven't decided the best way to do that yet. Currently I've been tossing around the idea of making the road selection "stack", and when you re-select an already selected road it adds +1 to the queue, triggering it to build the next 2 road types (or more). But that might get annoying, because it may require you to select the ground 5+ times to stack the selection from no road all the way up to the maximum road type.