1998 Mazda MPV Engine Oil is Leaking

Engine problem
1998 Mazda MPV V6 Two Wheel Drive Automatic 130000 miles

I noticed burning oil when I parked my car in the garage. I also noticed oil drops on the garage floor. Next day, the repair shop person stated that Valve Cover Gaskets were the problem and that should fix it. In the afternoon, I picked up the car and was assured that the car was test driven and everything is fixed. I drove a couple of miles and saw smoke coming out (from burning oil). Took it back. Now we could all see some oil towards the back side of the engine - from where the smoke was coming out.

The repair shop took the car for the day and next day claimed that the car was fixed. The car is still leaking oil. I have driven 200 miles over the weekend and it has leaked half a quart. I am taking it back to the repair shop tomorrow but am losing confidence in the shop.

The car runs fine. And I am checking the oil level after every drive. But the smoke is bothersome. The car could catch fire.

The leak seems to at the back side/bottom of the engine block - when viewed from the front of the car (with hood raised). There is no leakage signs on the top of the car.

What else could be looked at?
Are there any reputed repair shops in Austin, TX (zip code 78759)? The current repair shop - Crescent Automotive - has already charged $280.01 (to replace valve cover gaskets) and I have paid two visits and the guy seems to be sure there is no other problem (& he lied about having test driven the car both times).