Economical replacement rocker arm set?

You can see all the cam lobes from the top with the intake off. The bore scope might work.....it is just very limited in field of view and I am not sure you will get it past the pushrods.

One thing to do is pull a valve cover and push down on all the suspect lifters right after you shut down. Rotate the engine and put each one on the heel of the lobe (valve closed) and push down on it. It should not be soft but one with a bad check valve will be weak.

And, to verify it is #5 or #7, go back by each exhaust and likely closely to each pipe while idling. Often a weak lifter will cause a mild missing sound in that side of the exhaust.

And you're right on the thinner head gasket closing up the valvetrain. But a low base circle in the cam would counter that. That can only be known by looking at the rockers and shafts for wear, and then looking at the preloads.