Check for that manifold leak (or any other air leaking in AFTER your mass airflow sensor) by spraying some starter fluid at various spots around the intake manifold/other air tubing.. If the engine races while spraying, you are getting extraflow after the MAF sensor and this could account for your lean condition.
Side note... I have had similar experiences and if I could manage to never take a vehicle to a mechanic again, I would be a happy man.. It's not their vehicle and the vast, vast majority will not put the necessary time/diagnostics into the job and will just throw fixes at it until you go away (probably because you are now broke).

Good luck!!

JD