Leak was just a guess - it's easy to run to a terminal diagnosis but sometimes its the simplest explanation. I reckon there are two separate issues: oil leak/consumption and fuel consumption, not necessarily related.
What do we have?
1 massive fuel consumption that even the most lead-footed would struggle to achieve, around 15mpg.
2 no obvious fuel leaks, over cooling or lambda issues
3 no coolant use
4 high oil consumption but no grey/black smoke
Are you sure it's smoke and not water vapour? My 'wagon makes a bit is steam when the air is cold and damp or it's not used on long trips, but uses next to no coolant.
The TS engines do consume some oil, some more than others.
I'm not sure how a rocker cover leak translates to smoke out if the tailpipe though.
Apologies if none of that helps, just trying to be methodical (ish).