Smoke misfire low MPG

I have a ford focus 2005 1.6 tdci GHIA spec with 140k miles. The problem started when I heard engine knocking when I was travelling at 60 kmph on a rainy day. The engine knock was there for about 5 mins and the sound went. The car used to run fine except for the black smoke when accelerating. However from the next day when I started my car, it started differently, very sluggish and loads of blue white smoke. The sound was also different, sounded very rough. I checked the engine oil level to find the level at the edge. I quickly bought the oil and added about 2 liters. I took the car for servicing. The garage also replaced rocker cover thinking it was the breather leaking oil to air pipe. The next day with cold start it was the same. If I rev the engine for long, I get blue whitish smoke until black smoke comes up. The car runs with some misfires and after about 2 mins of driving runs okay, little sluggish and smoke after every idle(traffic lights/parking). The oil is decreasing heavily. I had about 43 MPG and now it has decreased to 33.

I have read that it could be valve stem seals, injector, egr and not sure which way to go. I am a student and I don't have a budget to fix everything. Does anyone have any idea on this please? Any input would be a great help. Thanks.