I had this exact issue (huge amount of white smoke only when accelerating onto the highway) that was fixed by head gasket replacement. My hypothesis was similar to your mechanic’s: oil spray from the front cover hitting the exhaust during hard acceleration while the engine was still cold-ish. It seemed to stop once the engine was fully warm, and friends driving behind me always said it seemed to be coming from under the car not the tailpipe so you could check that way?