Wy, I took a closer look...and you are correct, there is nothing wrong with it grammatically....unless you contest the use of "as it should seem" being used to mean "it seems".
A translation into layman's terms...
Quote:It is to avoid this plain confession of truth, as it should seem,
It seems that in order to avoid admitting to the previously mentioned fact,
Quote:that this imitation of masters-indeed, almost all imitation which implies a more regular and progressive method of attaining the ends of painting-
that this imitation of a master painter's works, or any artistic imitation which provides an easier or more methodical approach to painting,
Quote:has ever been particularly inveighed against with great keenness, both by ancient and modern writers.
has always been looked down upon by writers.
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And a summary...
This imitation, and more generally all imitations of master painters' works, which offer a much more procedural and methodical approach to painting, have always been looked down upon to hide this* simple truth.
* = refers to a statement in the preceeding sentence that the author considers a fact
In order to keep the precise original meaning, the comma after "works" should be removed. However, I felt like adding the implication that all imitations offer an easier and more methodical approach...because I believe it.
So oristar, your translation is in fact not valid.
It is worth noting the definition of "inveighed" which means essentially "looked down upon" (I didn't know it).