Perhaps I read too much into the quality of workmanship issue. Obviously the skulls don't prove anything if it would be a simple matter for someone to make a similarly convincing skull using lapidary equipment in the 18th century.
I think the problem I have with this line of reasoning is that it typically doesn't seem to cut both ways. The accepted theories are automatically assumed to be utterly devoid of any possibility of forgery, even when large parts of the evidence presented are susceptible to forgery.
I'm not sure I follow correctly what you're suggesting here, but if I am, then you're wondering why scientific archaeological methods don't encounter the same skepticism as questionable artifacts of little or no provenience which are also encumbered by fantastic claims or significance.
This is a legitimate question to which there are many answers. For one, artifacts without documented provenience are
immediately questionable and necessarily so. Where did it come from? What stratum? When? What position was it in in the ground? What are the over- and underlying strata? etc.
For another, archaeologists rarely work alone. They are generally working in teams and the excavation of an artifact is one that is painstakingly documented and eventually removed. If the artifact is of significance (i.e. intact ceramics, funerary goods, a cache of obsidian lithics, etc.) then there are going to be many photographs taken during the excavation, diagrams written, and witnesses/participants to removing it from the ground.
Moreover, the archaeologists themselves have deserved authority and legitimacy if by the mere fact that they are educated and trained. This, of course, doesn't eliminate human tendencies to be greedy, ambitious, or deceptive -but it does offer a bit more credibility than untrained and amateur looters and antiquarians who are motivated not by knowledge but by profit and sensation.
In addition, archaeologists seek to replicate and reproduce results as well as to make predictions that are confirmed -this is part of the scientific method. With this in mind, the discoveries of archaeology are often made available to peers who often have competing hypotheses that
they wish to see confirmed. This sort of peer review can be merciless in science and, particularly, in the field of archaeology as I've witnessed first hand. The knowledge of this alone keeps archaeologists on their toes and they are very careful about the claims they make. The hypothesis is tested and retested -they intentionally seek to find data that will fail their hypotheses before going public.