Wala bang vehicle speed sensor (VSS) ang 2000 Corolla? If the speedo cable was re-attached properly and still has problems, then it might be the VSS already.

Re: the oil leak, easy enough to check where the leak is coming from. Just have them lift the car (via a lifter), and check the seals if it's wet around the edges. Maybe they used a low-quality sealant or non-original gasket. (Too much sealant is also bad). But I'd rather have this done at your friendly gas station with a service bay, kasi sablay naman pala sila the 2nd (and 3rd and 4th time) around, hehehe.