Australian Car Repair Issues? Ask an Auto Mechanic for Answers ASAP
I would say your first mechanic is at fault here.
Unlikely to need new injectors if it was running fine prior to the rocker cover gasket job.
The injector seat washers need to be replaced when the injectors are removed, and the injectors need to be torqued to spec when refitted.
The injector bodies and the tubes in the head need to be spotlessly clean on reassembly.
Hope this helps.
Do you need any further help?
I hope my advice has been of assistance to you.