Firstly I would be checking the servicing dealership's work as it is a bit of a coincidence that the fault only started on the way home from service.
Secondly, stretched chains resulting in missfire usually throw up a code.
Finally, a new genuine Nissan 3 x chain kit, slippers, guides and tensioners costs circa £350.00.
20 hours labour is ample for this job so depending on hourly labour charge, £1800.00 should cover it.
The job can be completed with engine out or insitu.
If you remove the front bumper, radiator support panel, radiator and condenser then access to the front chain cover becomes much easier without engine removal, and is doable in 10 hours to replace the chains etc with engine insitu.
As has already been said, best to get a second/third opinion from a VQ35DE specialist.
Alex.